testing: use testenv.Executable

Note that this changes some nuances of how the tests work:
 - some tests had a fallback to using os.Args[0], which is removed;
 - some tests skipped (rather than failed) the test upon getting an
   error from os.Executable.

I think these changes are not practically relevant.

Change-Id: I0655add6d959a8b7e3359f94c38203aa06e8f490
Reviewed-on: https://go-review.googlesource.com/c/go/+/609303
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/testing/flag_test.go

@@ -28,11 +28,7 @@ func TestFlag(t *testing.T) {
 		flag := flag
 		t.Run(flag, func(t *testing.T) {
 			t.Parallel()
-			exe, err := os.Executable()
+			cmd := exec.Command(testenv.Executable(t), "-test.run=^TestFlag$", "-test_flag_arg="+flag)
-			if err != nil {
-				exe = os.Args[0]
-			}
-			cmd := exec.Command(exe, "-test.run=^TestFlag$", "-test_flag_arg="+flag)
 			if flag != "" {
 				cmd.Args = append(cmd.Args, flag)
 			}

src/testing/helper_test.go

@@ -23,15 +23,9 @@ func TestTBHelper(t *testing.T) {
 		return
 	}
 
-	testenv.MustHaveExec(t)
 	t.Parallel()
 
-	exe, err := os.Executable()
+	cmd := testenv.Command(t, testenv.Executable(t), "-test.run=^TestTBHelper$")
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	cmd := testenv.Command(t, exe, "-test.run=^TestTBHelper$")
 	cmd = testenv.CleanCmdEnv(cmd)
 	cmd.Env = append(cmd.Env, "GO_WANT_HELPER_PROCESS=1")
 	out, _ := cmd.CombinedOutput()
@@ -66,15 +60,9 @@ func TestTBHelperParallel(t *testing.T) {
 		return
 	}
 
-	testenv.MustHaveExec(t)
 	t.Parallel()
 
-	exe, err := os.Executable()
+	cmd := testenv.Command(t, testenv.Executable(t), "-test.run=^TestTBHelperParallel$")
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	cmd := testenv.Command(t, exe, "-test.run=^TestTBHelperParallel$")
 	cmd = testenv.CleanCmdEnv(cmd)
 	cmd.Env = append(cmd.Env, "GO_WANT_HELPER_PROCESS=1")
 	out, _ := cmd.CombinedOutput()

src/testing/testing_test.go

@@ -440,12 +440,7 @@ func runTest(t *testing.T, test string) []byte {
 
 	testenv.MustHaveExec(t)
 
-	exe, err := os.Executable()
+	cmd := testenv.Command(t, testenv.Executable(t), "-test.run=^"+test+"$", "-test.bench="+test, "-test.v", "-test.parallel=2", "-test.benchtime=2x")
-	if err != nil {
-		t.Skipf("can't find test executable: %v", err)
-	}
-
-	cmd := testenv.Command(t, exe, "-test.run=^"+test+"$", "-test.bench="+test, "-test.v", "-test.parallel=2", "-test.benchtime=2x")
 	cmd = testenv.CleanCmdEnv(cmd)
 	cmd.Env = append(cmd.Env, "GO_WANT_HELPER_PROCESS=1")
 	out, err := cmd.CombinedOutput()
@@ -674,14 +669,7 @@ func TestRaceBeforeParallel(t *testing.T) {
 }
 
 func TestRaceBeforeTests(t *testing.T) {
-	testenv.MustHaveExec(t)
+	cmd := testenv.Command(t, testenv.Executable(t), "-test.run=^$")
-
-	exe, err := os.Executable()
-	if err != nil {
-		t.Skipf("can't find test executable: %v", err)
-	}
-
-	cmd := testenv.Command(t, exe, "-test.run=^$")
 	cmd = testenv.CleanCmdEnv(cmd)
 	cmd.Env = append(cmd.Env, "GO_WANT_RACE_BEFORE_TESTS=1")
 	out, _ := cmd.CombinedOutput()