TOOLS-4263 Convert mongorestore namespace mapping tests to Go

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Adds integration/dumprestore/namespace_test.go: six DumpRestoreSuite methods, fourteen cases in total, covering how mongorestore targets and rewrites namespaces. Every fixture document's _id records the namespace it was inserted into, because the JS tests mostly asserted document counts and a count cannot tell a correct restore apart from one that swapped two same-named collections between databases. Every restore also asserts an exact insert count, since mongorestore returns a nil error for a restore that found nothing to do.

JS -> Go mapping (all Go tests live in integration/dumprestore/namespace_test.go):

• test/qa-tests/jstests/restore/different_collection.js -> TestRestoreIntoDifferentCollection (integration/dumprestore/namespace_test.go) - a single collection's bson file is restored through --db and --collection into a different collection in the same database, into the same collection name in a different database, and into a different collection in a different database, with the documents landing only in the requested target each time.
• test/qa-tests/jstests/restore/different_db.js -> TestRestoreIntoDifferentDB (integration/dumprestore/namespace_test.go) - a whole database directory is redirected with --db, and then the same dump is remapped with --nsFrom '$db$.$collection$' --nsTo 'otherdest.$db$_$collection$' so that both halves of the namespace are rewritten and the source database name ends up folded into the destination collection names.
• test/qa-tests/jstests/restore/namespaces.js -> TestRestoreNamespaceMappings (integration/dumprestore/namespace_test.go) - a three-database fixture is restored through a mapping that flattens every source database into one destination, combined in turn with --nsExclude, --nsInclude, and --excludeCollectionsWithPrefix, and then restored again with two --nsFrom/--nsTo pairs that exchange a pair of databases. The swap case is the interesting one: mongorestore resolves every mapping against the original namespace and returns on the first match rather than chaining them, so applying the two pairs in sequence would send everything to one database, and the encoded _ids catch that. Each restored collection is also checked to have kept the index built over a field named for its source namespace, asserting the index key rather than just its name.
• test/qa-tests/jstests/restore/multiple_dbs.js -> TestRestoreMultipleDBs (integration/dumprestore/namespace_test.go) - two databases that share a collection name round-trip together and each ends up with exactly its own documents.
• test/qa-tests/jstests/restore/partial_restore.js -> TestPartialRestore (integration/dumprestore/namespace_test.go) - from a dump spanning two databases, restoring one database and then one collection leaves every namespace that was not asked for absent rather than merely empty.
• test/legacy42/jstests/tool/dumprestore4.js -> TestRestoreIntoDifferentDBBuildsIndexes (integration/dumprestore/namespace_test.go) - covers SERVER-2186: restoring a dump into a differently named database gives the destination the source's indexes, while a same-named database that happens to exist alongside it is left untouched.

One implementation note worth recording: mongodump accepts a single --db and has no --nsInclude, so a dump spanning several databases has to be assembled one database at a time into a shared root, which withMultiDBDump does. Dumping everything instead would sweep admin and config into the dump and then into the restores.

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• TOOLS-4263 Convert mongodump oplog consistency tests to Go #1104
• TOOLS-4263 Convert mongodump query and data behavior tests to Go #1102
• TOOLS-4263 Convert mongodump flag validation tests to Go #1098
• TOOLS-4263 Convert mongorestore users and roles temp collection tests to Go #1097
• TOOLS-4263 Convert mongorestore drop, path and stdin tests to Go #1096
• TOOLS-4263 Convert mongorestore namespace mapping tests to Go #1093 👈 (View in Graphite)
• TOOLS-4263 Convert mongorestore bulk sizing tests to Go #1091
• TOOLS-4263 Convert mongorestore index round-trip tests to Go #1074
• TOOLS-4263 Convert mongorestore collection-options and metadata tests to Go #1073
• TOOLS-4263 Convert mongorestore replica-set oplog-replay tests to Go #1049
• TOOLS-4263 Convert mongorestore oplog-replay edge-case tests to Go #1060
• TOOLS-4263 Convert mongorestore invalid-input tests to Go #1046 : 1 other dependent PR (#1048 )
• TOOLS-4263 Delete JS tests fully covered by existing Go tests #1041
• TOOLS-4263 Add a Go test that we preserve indexes when restoring with a non-admin user on a per-data basis #1047
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