unix: use os.Executable rather than os.Args[0] in tests

Change-Id: I67a063d747c6e34dcd0292fdb3a9a0d965a6e133
Reviewed-on: https://go-review.googlesource.com/c/sys/+/607875
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

unix/openbsd_test.go

@@ -43,7 +43,7 @@ func init() {
 // testCmd generates a proper command that, when executed, runs the test
 // corresponding to the given key.
 func testCmd(procName string) (*exec.Cmd, error) {
-	exe, err := filepath.Abs(os.Args[0])
+	exe, err := os.Executable()
 	if err != nil {
 		return nil, err
 	}