[release-branch.go1.15] syscall: use MustHaveExec in TestExec

For #41702
For #41704

Change-Id: Ib2b15e52aa1fef2f5e644b316c726150252fa9f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/262738
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>
(cherry picked from commit 11cfb48)
Reviewed-on: https://go-review.googlesource.com/c/go/+/264020

Author: ianlancetaylor

src/syscall/exec_unix_test.go

@@ -247,6 +247,7 @@ func TestInvalidExec(t *testing.T) {
 
 // TestExec is for issue #41702.
 func TestExec(t *testing.T) {
+	testenv.MustHaveExec(t)
 	cmd := exec.Command(os.Args[0], "-test.run=TestExecHelper")
 	cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=2")
 	o, err := cmd.CombinedOutput()