chore: rename e2etest tag to mock_github (#2827)

Renaming `e2etest` tag to `mock_github` for clarity.

I find it confusing that we have both `e2e` tag and `e2etest` tag. 
The `e2e` tag is used for end to end test and is a widely used Go
convention.
The `e2etest` tag was added in #2228 to switch between a mock
implementations of repository for e2e test to avoid real API calls and
the real implementation for production code.

Author: zhumin8

e2e_test.go

@@ -41,6 +41,8 @@ import (
 var nestedCommitRegex = regexp.MustCompile(`(?s)BEGIN_NESTED_COMMIT\n(.*?)\nEND_NESTED_COMMIT`)
 var conventionalCommitRegex = regexp.MustCompile(`^(feat|fix|docs|chore): (.+)$`)
 
+const mockGithubTag = "mock_github"
+
 func TestRunGenerate(t *testing.T) {
 	const (
 		initialRepoStateDir = "testdata/e2e/generate/repo_init"
@@ -121,7 +123,7 @@ func TestRunGenerate(t *testing.T) {
 			defer server.Close()
 			cmdArgs := []string{
 				"run",
-				"-tags", "e2etest",
+				"-tags", mockGithubTag,
 				"github.com/googleapis/librarian/cmd/librarian",
 				"generate",
 				fmt.Sprintf("--api=%s", test.api),
@@ -211,7 +213,7 @@ func TestCleanAndCopy(t *testing.T) {
 	cmd := exec.Command(
 		"go",
 		"run",
-		"-tags", "e2etest",
+		"-tags", mockGithubTag,
 		"github.com/googleapis/librarian/cmd/librarian",
 		"generate",
 		fmt.Sprintf("--api=%s", apiToGenerate),
@@ -526,7 +528,7 @@ func TestReleaseStage(t *testing.T) {
 
 			cmdArgs := []string{
 				"run",
-				"-tags", "e2etest",
+				"-tags", mockGithubTag,
 				"github.com/googleapis/librarian/cmd/librarian",
 				"release",
 				"stage",

internal/librarian/repository.go

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build !e2etest
+//go:build !mock_github
 
 // This file contains the production implementations for functions that get
 // GitHub repository details.

internal/librarian/repository_mock.go

@@ -12,10 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build e2etest
+//go:build mock_github
 
 // This file contains mock implementations of repository getters for use in
-// end-to-end tests. It is compiled only when the 'e2e-test' build tag is specified.
+// end-to-end tests. It is compiled only when the 'mock_github' build tag is specified.
 
 package librarian