tests: api: allow both case of pathsep

tests/api.c

 	(void)argc; // unused parameter
 
 	char *progdir = strdup(argv[0]);
-	char *dir = strrchr(progdir, PATHSEP);
-	if (dir != NULL) *dir = 0;
+
+	// Path separator on windows may be both '/' and '\'
+	char *dir;
+	if ((dir = strrchr(progdir, PATHSEP)) != NULL) *dir = 0;
+	else if ((dir = strrchr(progdir, '/')) != NULL) *dir = 0;
 
 	test_espeak_terminate_without_initialize();
 	test_espeak_initialize();