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
11 jobs for test-CompileFeatures-c11-range
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