Tests: Only run fCustomSyntaxMac with FC_OS_MACOSX

Originally this test file simply defined the appropriate macro, but
since macros leak that can have unexpected side effects on other tests.
It's safer to simply only run that test on the Mac.

tests/src/App/ProgramOptionsUtilities.cpp

@@ -15,9 +15,11 @@ TEST(ProgramOptionsUtilitiesTest, fCustomSyntaxLookup)
 };
 TEST(ProgramOptionsUtilitiesTest, fCustomSyntaxMac)
 {
+#if defined(FC_OS_MACOSX)
     stringPair res {customSyntax("-psn_stuff")};
     stringPair exp {"psn", "stuff"};
     EXPECT_EQ(res, exp);
+#endif
 };
 TEST(ProgramOptionsUtilitiesTest, fCustomSyntaxWidgetCount)
 {