command/pr: PRs for Deleted Files Extraneously Inspected for Tests

[bflad]

Description

Example: https://github.com/terraform-providers/terraform-provider-aws/pull/11565/files

$ tctest pr 11565
Discovering tests for pr #11565 (https://github.com/terraform-providers/terraform-provider-aws/pull/11565)...
WARN[2020-01-16 01:01:37] unable to fetch tests from file (aws/tagsGeneric_test.go): unable to get content from https://raw.githubusercontent.com/terraform-providers/terraform-provider-aws/cfd95109f1804e21bef3156d324fdb36f8cbccea/aws/tagsGeneric_test.go: HTTP status NOT OK: 404
    TestAccAwsBackupPlan
triggering refs/pull/11565/merge for (TestAccAwsBackupPlan)...
  bflad@--OMITTED--#Aws_ProviderAwsAlternate
  build 92522 queued! (https://--OMITTED--/viewQueued.html?itemId=92522)