Validate PEGTL tests when no filesystem is configured

[uilianries]

I merged the PR #350 to a new branch created from 3.x, and added a new new validation in Actions to check that we don't break tests when running without filesystem.

The filesystem has a specific test test_filesystem.cpp.in that should be activated only when PEGTL_USE_FILESYSTEM is active. So this new regression test checks in case we break something when changing filesystem.

Related to #350

[ColinH]

Great, thanks, you can merge this in to the 3.x branch when you think it's ready, and I'll take care of removing the change and boost::filesystem support from the main branch RSN.

[uilianries]

@ColinH I see Github Action is failing due missing compilers. I'll take a look on this. It seems like the latest version of Ubuntu distributed by Github does not support older compiler versions.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (cf639f7) 99.70% compared to head (191fe2e) 99.70%.

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Additional details and impacted files

@@           Coverage Diff           @@
##              3.x     #351   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files         248      250    +2     
  Lines        5045     5096   +51     
=======================================
+ Hits         5030     5081   +51     
  Misses         15       15           

see 16 files with indirect coverage changes

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[uilianries]

Clang Tidy failed, but I'll ignore it for now, as it could be customized.