Add tests for stdlib pinnings

Some migrators might need to set a higher `c_stdlib_version`, c.f. https://github.com/conda-forge/conda-forge.github.io/issues/2467 However, we really need to avoid the bot breaking our stdlib-pinning by naïvely closing such migrations (c.f. #5559 #7304). I think we should add some tests that ensure that the content of https://github.com/conda-forge/conda-forge-pinning-feedstock/blob/3a222220ffab33116c5601592322e0cd67352493/recipe/conda_build_config.yaml#L21-L24 contains what's expected. This is because changes like ```diff -c_stdlib_version: # [unix] - - 2.17 # [linux] - - 10.13 # [osx and x86_64] - - 11.0 # [osx and arm64] +c_stdlib_version: # [osx and x86_64] + - 10.14 # [osx and x86_64] ``` in https://github.com/conda-forge/conda-forge-pinning-feedstock/commit/e04df46a5f81872d1213784ebacf3118aad6df92 actually cause `{{ stdlib("c") }}` to pull in to pull in the highest available version, which (as of today) leads to packages requiring `__glibc >=2.34`, which is waaaaaay newer than what we currently have or want (see https://github.com/conda-forge/conda-forge.github.io/issues/2383). Duplicating this configuration in some pre-commit tests is annoying duplication, but given the blast radius of the bot messing something up in that configuration, we really should add a safety net IMO.