Features: Fix CompileFeatures test for C non-features

Add a "feature_test.c" file corresponding to "feature_test.cpp" but for
the C language.  This source will be needed by C_non_features entries.

Tests/CompileFeatures/feature_test.c

@@ -0,0 +1,10 @@
+
+#define STRINGIFY_IMPL(X) #X
+#define STRINGIFY(X) STRINGIFY_IMPL(X)
+
+#include STRINGIFY(TEST)
+
+int main(void)
+{
+  return 0;
+}