chore(auth/grpctransport): remove env impact on direct path tests

[westarle]

This clears the GOOGLE_CLOUD_DISABLE_DIRECT_PATH environment variable in TestLogDirectPathMisconfig tests to prevent failures when the environment variable is set in the environment running the tests. This fixes a nuisance test error when running tests on cloudtop.

[gemini-code-assist]

Code Review

This PR correctly makes the direct path tests independent of the environment by unsetting GOOGLE_CLOUD_DISABLE_DIRECT_PATH. To improve the test suite's maintainability, I've suggested refactoring the three modified tests, which share a lot of boilerplate, into a single table-driven test. This would reduce code duplication and make future modifications easier.

[gemini-code-assist]

This line is also added to TestLogDirectPathMisconfigWrongCredential and TestLogDirectPathMisconfigNotOnGCE. Since these three tests are very similar, consider refactoring them into a single table-driven test. This would reduce code duplication and improve maintainability. You would only need to call t.Setenv once in the parent test.

Here's an example of how it could look:

func TestLogDirectPathMisconfig(t *testing.T) {
	t.Setenv(disableDirectPathEnvVar, "")

	testCases := []struct {
		name        string
		opts        *Options
		creds       *google.Credentials
		onGCE       bool
		wantWarning string
	}{
		{
			name: "DirectPathNotSet",
			opts: &Options{InternalOptions: &InternalOptions{
				EnableDirectPathXds: true,
			}},
			wantWarning: directPathNotSetWarning,
		},
		{
			name: "WrongCredential",
			opts: &Options{InternalOptions: &InternalOptions{
				EnableDirectPathXds: true,
				EnableDirectPath:    true,
			}},
			creds: &google.Credentials{
				TokenSource: oauth2.StaticTokenSource(&oauth2.Token{}),
			},
			wantWarning: wrongCredsWarning,
		},
		{
			name: "NotOnGCE",
			opts: &Options{InternalOptions: &InternalOptions{
				EnableDirectPath:    true,
				EnableDirectPathXds: true,
			}},
			creds: &google.Credentials{
				ProjectID: "project-id",
			},
			wantWarning: notOnGCEWarning,
		},
	}

	for _, tc := range testCases {
		t.Run(tc.name, func(t *testing.T) {
			var buf bytes.Buffer
			log.SetOutput(&buf)
			defer log.SetOutput(os.Stderr)

			logDirectPathMisconfig(tc.opts, tc.creds, tc.onGCE)

			if got := buf.String(); !strings.Contains(got, tc.wantWarning) {
				t.Errorf("got %q, want substring %q", got, tc.wantWarning)
			}
		})
	}
}

You would then remove TestLogDirectPathMisconfigDirectPathNotSet, TestLogDirectPathMisconfigWrongCredential, and TestLogDirectPathMisconfigNotOnGCE.