Include most of the test-folder in the LGTM report (PR 13772 follow-up)

[Snuffleupagus]

Given that PR #13772 seem to have worked as intended, it probably cannot hurt to enable the LGTM report for most of the remaining code in the test-folder. (The one remaining exception is code which originated outside of the PDF.js project.)

Please note: We'll need to land this patch to actually see any difference in the LGTM results.

[Snuffleupagus]

I'm going to make an exception and land this without waiting for review, since:

• We cannot tell if this actually works as intended, without landing the patch.
• The patch is very small and simple, and only applies to the LGTM-integration.
• Finally, and most importantly, none of this code applies to the built PDF.js library and cannot possibly affect rendering, the viewer, or any other even remotely critical functionality.