interp: skip TestRunnerTerminalExec/Pseudo on Mac

While we can't reproduce the failures on Mac laptops, GitHub's Mac
machines fail this test pretty often. We can't have CI be that flaky.

Updates #513.

interp/terminal_test.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"os"
 	"os/exec"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -112,6 +113,9 @@ func TestRunnerTerminalExec(t *testing.T) {
 	for _, test := range tests {
 		test := test
 		t.Run(test.name, func(t *testing.T) {
+			if test.name == "Pseudo" && runtime.GOOS == "darwin" {
+				t.Skipf("too flaky on GitHub's Mac machines; see https://github.com/mvdan/sh/issues/513")
+			}
 			t.Parallel()
 
 			cmd := exec.Command(os.Getenv("GOSH_PROG"),