Fix Acceptance Test For TestAccGithubUserSshKey_basic And TestAccGithubRepositoryDeployKey_basic

[jcudit]

Acceptance test runs have been failing for TestAccGithubUserSshKey_basic and TestAccGithubRepositoryDeployKey_basic .

I vaguely remember one failing before the other started and now are currently failing for the master branch.

=== CONT  TestAccGithubRepositoryDeployKey_basic
--- FAIL: TestAccGithubRepositoryDeployKey_basic (7.18s)
    testing.go:654: Step 0 error: errors during apply:

        Error: POST https://api.github.com/repos/terraformtesting/acctest-4xducydblu/keys: 422 Validation Failed [{Resource:PublicKey Field:key Code:custom Message:key is already in use}]

          on /var/folders/t1/p9cyhv6s2wg4g8s_x6zkpz_m0000gn/T/tf-test131430109/main.tf line 6:
          (source code not available)


FAIL
FAIL	github.com/terraform-providers/terraform-provider-github/github	7.461s

=== CONT  TestAccGithubUserSshKey_basic
--- FAIL: TestAccGithubUserSshKey_basic (0.73s)
    testing.go:654: Step 0 error: errors during apply:

        Error: POST https://api.github.com/user/keys: 422 Validation Failed [{Resource:PublicKey Field:key Code:custom Message:key is already in use}]

          on /var/folders/t1/p9cyhv6s2wg4g8s_x6zkpz_m0000gn/T/tf-test744524835/main.tf line 2:
          (source code not available)


FAIL
FAIL	github.com/terraform-providers/terraform-provider-github/github	1.023s

I suspect a test suite run was halted, leaving around acceptance test fixtures that are reused on subsequent runs. A solution may involve using unique values instead of having to clean up successfully 100% of the time.

/cc https://github.com/terraform-providers/terraform-provider-github/pull/411