Close test databases to avoid leaks

Close local and fixture test databases to avoid database leaks across Room test suites:
* DispatchersTest.publicUseConnectionConfiguredDispatcher
* SuspendingQueryTest.withWriteTransaction_reentrant_nested_exception
* NullabilityAwareTypeConversionTest
* PassthroughConnectionPoolTest.reusingConnection_instanceCheck
* TestDatabaseTest and MailDaoTest (autovaluetestapp)
* BaseQueryTest (multiplatformtestapp)
* LimitOffsetPagingSourceTest.load_usesQueryExecutor

Bug: 539512913
Test: ./gradlew \
  :room3:integration-tests:room-testapp-kotlin:connectedAndroidTest \
  :room3:integration-tests:room-testapp-autovalue:connectedAndroidTest \
  :room3:integration-tests:room-testapp-multiplatform:connectedAndroidDeviceTest
Change-Id: Ie922252d330d0acdc23e81791d3350ba9e1f6744