Ensure that namespaced github auth mounts are destroyed

[benashz]

• add regression test for namespaced mount
• normalize related tests

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Closes #1635

Release note for CHANGELOG:

Output from acceptance testing:

 $ make testacc-ent SKIP_VAULT_NEXT_TESTS=1 TESTARGS='-v -test.run TestAccGithubAuthBackend -test.count=1'
make testacc TF_ACC_ENTERPRISE=1
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v -test.run TestAccGithubAuthBackend -test.count=1 -timeout 30m ./...

2022/10/13 13:08:35 [INFO] Using Vault token with the following policies: root
=== RUN   TestAccGithubAuthBackend_basic
--- PASS: TestAccGithubAuthBackend_basic (1.83s)
=== RUN   TestAccGithubAuthBackend_ns
--- PASS: TestAccGithubAuthBackend_ns (1.84s)
=== RUN   TestAccGithubAuthBackend_tuning
--- PASS: TestAccGithubAuthBackend_tuning (1.66s)
=== RUN   TestAccGithubAuthBackend_description
--- PASS: TestAccGithubAuthBackend_description (1.68s)
=== RUN   TestAccGithubAuthBackend_importTuning
--- PASS: TestAccGithubAuthBackend_importTuning (1.23s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     8.837s

[vinay-gopalan]

Looks great!