test(check.rb): add #extra_args instruction

[tueda]

This is probably the simplest approach to resolve the issue (testing tform -w2) discussed in (#520 (comment)).

This patch adds a new special instruction #extra_args that can be used to add extra command-line arguments to be passed to FORM. For example

#extra_args -w2

leads to run tform -w4 -w2 1.frm for testing TFORM.

Instead of adding #extra_args, other options would be:

1. Run all TFORM tests with -w2 and then with -w4.
2. Add

   #threads 2,4
   

   to run TFORM tests with -w2 and then with -w4.

[jodavies]

I would say it is probably best to run the entire set of tests with both w2 and w4. I am not sure on the limits of github's runners. The valgrind tests are taking the majority of the time at the moment (and those should probably be run with both w2 and w4 as well, since different code runs...). #520 manifests even for very trivial input. Maybe there is even something in the current set of tests that doesn't work with w2...

[coveralls]

coverage: 48.768% (+0.004%) from 48.764%
when pulling c34be2d on tueda:test-extra_args
into 87b4bcf on vermaseren:master.

[tueda]

You are right. Adding -w2 actually caused many test failures... (I'll create a PR).

[tueda]

This patch is not needed for now, but we may revisit it later.