tests: Simplify multi-region test logic

[radeksimko]

There is a couple of benefits we gain from this refactoring:

• less duplicated code (especially duplicate providerFactories := map[string]terraform.ResourceProviderFactory blocks in each test)
• it should be now more obvious from looking at the test body which region is each function checking
• more precise checks with no false positives and less API calls - i.e. we no longer check resource's existence via each applicable (AWS) provider, but instead pick only the one for the right region
• nil checks are kept in a single place, no more spreading around tests. Test now always gets a valid provider pointer.

Test results

=== RUN   TestAccAWSVPCPeeringConnection_region
--- PASS: TestAccAWSVPCPeeringConnection_region (29.16s)
=== RUN   TestAccAWSS3Bucket_ReplicationExpectVersioningValidationError
--- PASS: TestAccAWSS3Bucket_ReplicationExpectVersioningValidationError (32.11s)
=== RUN   TestAccAWSVPCPeeringConnectionAccepter_differentRegion
--- PASS: TestAccAWSVPCPeeringConnectionAccepter_differentRegion (36.30s)
=== RUN   TestAccAWSS3Bucket_ReplicationWithoutStorageClass
--- PASS: TestAccAWSS3Bucket_ReplicationWithoutStorageClass (48.38s)
=== RUN   TestAccAWSRoute53Zone_private_region
--- PASS: TestAccAWSRoute53Zone_private_region (68.29s)
=== RUN   TestAccAWSS3Bucket_Replication
--- PASS: TestAccAWSS3Bucket_Replication (74.84s)
=== RUN   TestAccAWSRoute53ZoneAssociation_region
--- PASS: TestAccAWSRoute53ZoneAssociation_region (105.09s)
=== RUN   TestAccAWSInstance_multipleRegions
--- PASS: TestAccAWSInstance_multipleRegions (232.98s)
=== RUN   TestAccAWSRoute53Zone_forceDestroy
--- PASS: TestAccAWSRoute53Zone_forceDestroy (343.23s)

[bflad]

This is awesome! Thanks for the cleanup.

[ghost]

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