gitannex: run the end to end test functions in parallel

The subtests within each end to end test function already run in
parallel, but the three test functions themselves ran one after
another.

Marking the functions parallel lets all their subtests overlap.

cmd/gitannex/e2e_test.go

@@ -239,6 +239,7 @@ func skipE2eTestIfNecessary(t *testing.T) {
 // parameters like repo layouts, and runtime may suffer from a combinatorial
 // explosion.
 func TestEndToEnd(t *testing.T) {
+	t.Parallel()
 	skipE2eTestIfNecessary(t)
 
 	for _, mode := range allLayoutModes() {
@@ -273,6 +274,7 @@ func TestEndToEnd(t *testing.T) {
 // to git-annex-remote-rclone-builtin and verify that annexed files remain
 // accessible.
 func TestEndToEndMigration(t *testing.T) {
+	t.Parallel()
 	skipE2eTestIfNecessary(t)
 
 	if _, err := exec.LookPath("git-annex-remote-rclone"); err != nil {
@@ -330,6 +332,7 @@ func TestEndToEndMigration(t *testing.T) {
 // remote are present on the other. Similarly, test that files deleted from one
 // are removed on the other.
 func TestEndToEndRepoLayoutCompat(t *testing.T) {
+	t.Parallel()
 	skipE2eTestIfNecessary(t)
 
 	if _, err := exec.LookPath("git-annex-remote-rclone"); err != nil {