Update: This PR depends on:

• Enhance check.rb (TESTFILEDIR and others) #743
• Implement qgraf-comptatible keywords for FORM interface #744

The former may not be needed depending on how we treat the cache files.

TODO:

• Move the test file (probably) to check/extra. Tests should be fast enough. Put them in check/diagrams.frm. The core routines moved to check/diagrams/diagrams.h.
• Define the standard test set. For example, it may include 2-loop amplitudes with commonly used options, like:
  • 0
  • `ONEPARTI_'+`FLOOP_'
• Add reference-output cache files for the standard test set.
• Add optional thorough tests that don't run on CI (possibly in check/diagrams).

This PR introduces tests for comparing the diagram generator with QGRAF. The core routines also support FeynGraph, which could be helpful when we implement and test vacuum diagrams in the future (#485). For now, the test cases in the file cover only simple 1-loop diagrams and compare them with QGRAF.

The code contains two #pipe preprocessor instructions. One checks whether a cache file exists (see also #240). The other checks the FeynGraph version, for which I couldn't find any way to get it from the result using a Jinja template.

Open questions:

• where to place this file (check, check/extra or check/diagrams)
• whether to use the cache and where to store cache files
• which test cases should be the defaults if we put this file in check

The CanonicalizeDiagrams procedure may perform better if spectators are used in a smarter way.