Use a more common locale, en_US.UTF-8 is not always present.

Refs https://code.wildfiregames.com/D2101
Thread:
https://wildfiregames.com/forum/topic/28209-binariessystemtest-fmtsprintf-f-with-separator/?tab=comments#comment-396875

This was SVN commit r24300.

source/ps/tests/test_fmt.h

@@ -25,7 +25,7 @@ public:
 	void test_basic()
 	{
 		// Make test behave independent of current host locale
-		char* old = setlocale(LC_ALL, "en_US.UTF-8");
+		char* old = setlocale(LC_ALL, "C");
 
 		TS_ASSERT_EQUALS(fmt::sprintf("abc"), "abc");