TOOLS-4263 Convert mongorestore users and roles temp collection tests to Go

[autarch]

Extends TestRestoreUsersOrRoles in integration/dumprestore/roundtrip_test.go with two cases covering how mongorestore handles the temporary collections it stages users and roles in, and deletes the two JS tests they replace.

JS -> Go mapping:

• test/qa-tests/jstests/restore/nonempty_temp_users.js -> the "with a nonempty temp users collection" case of TestRestoreUsersOrRoles (integration/dumprestore/roundtrip_test.go) - mongorestore stages users in admin.tempusers before merging them into the real user collection, so a document left there by an interrupted earlier run must neither derail the restore nor survive it. The test seeds that collection with a junk document before restoring, then asserts the restore succeeded, the temp collections are gone, and the user and role from the dump actually exist.
• test/qa-tests/jstests/restore/users_and_roles_temp_collections.js -> the "with custom temp collection names" case of TestRestoreUsersOrRoles (integration/dumprestore/roundtrip_test.go) - this is the first coverage anywhere of --tempUsersColl and --tempRolesColl. The restore is run with custom names for both, and the test asserts those custom collections are cleaned up afterwards and that the user and role were restored.

Both JS tests asserted only the tool's exit code; these assert that the user and role exist afterwards, which is what the temp-collection machinery is in service of. The fixture user and role are dropped before each case, because BeforeTest deliberately leaves the admin database alone and that is where they live.

[autarch]

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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 👈 (View in Graphite)
• TOOLS-4263 Convert mongorestore drop, path and stdin tests to Go #1096
• TOOLS-4263 Convert mongorestore namespace mapping tests to Go #1093
• 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
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