Ignore warnings in regression tests

[xbauch]

Includes a regression test that generates a warning.

Reasoning: System package contains export/import pragmas that we want to ignore
but now about. Our regression test was sensitive to all gnat2goto output,
including warnings.

[NlightNFotis]

LGTM. There's a small typo in the commit message (now was supposed to be know?). Also, I feel like this should be documented somewhere else as well.

[chrisr-diffblue]

This feels like a very blunt instrument at the moment, because it basically filters out all warnings - but we probably want to know if certain warnings suddenly appear.... I also can't see where we would see these warnings in CI?

Can we not just update all the test.out files to include the warnings that we know about? I know that would be a pain to maintain, but it means there is a record (in the commit history) of what warnings we are aware of, and any unexpected new warnings will show up.

[tjj2017]

The warnings are generated by the Import pragmas. A warning is given by gnat2goto that non Ada languages are not supported. However, in the case of the Import pragmas in System and its child packages, the calling convention is "intrinsic" which indicates it is a subprogram built into the compiler (see ALRM 6.3.1) and as such I think can be safely ignored without issuing a warning.

I have a draft modification which does this but have not committed it yet. I wanted to get other people's views.

[xbauch]

Unnecessary now that #237 is merged (and does not produce any warnings).