Fix unit tests to succeed on macOS Catalina. (#796)

Catalina made parts of the OS read-only, so writing to stderr/out will now return a read-only error as opposed to Linux.

writer_test.go

@@ -72,8 +72,8 @@ func TestOpen(t *testing.T) {
 				"open /baz/quux: no such file or directory",
 			},
 		},
-		{[]string{"file:///stderr"}, []string{"open /stderr: permission denied"}},
-		{[]string{"file:///stdout"}, []string{"open /stdout: permission denied"}},
+		{[]string{"file:///stderr"}, []string{"open /stderr:"}},
+		{[]string{"file:///stdout"}, []string{"open /stdout:"}},
 		{[]string{"file://host01.test.com" + tempName}, []string{"empty or use localhost"}},
 		{[]string{"file://rms@localhost" + tempName}, []string{"user and password not allowed"}},
 		{[]string{"file://localhost" + tempName + "#foo"}, []string{"fragments not allowed"}},