Tests: Make CompileFeatures C dialect check consistent with impl

`Modules/CMakeCCompilerId.c.in` will report the C dialect as 11 whenever
`__STDC_VERSION__` indicates *at least* C 11.  Make the test consistent
with this.  We already do this for the C++ case.

Fixes: #17740

Tests/CompileFeatures/default_dialect.c

@@ -1,6 +1,6 @@
 
 #if DEFAULT_C11
-#if __STDC_VERSION__ != 201112L
+#if __STDC_VERSION__ < 201112L
 #error Unexpected value for __STDC_VERSION__.
 #endif
 #elif DEFAULT_C99