Test the real XGAP under Xvfb, with coverage

[fingolfin]

tst/testall.g talks to a GAP in package mode without any window, so it can neither open a sheet nor reach the C code. This adds tst-gui/, whose tests run inside the XGAP binary; CI starts it under Xvfb.

• sheet.tst: graphic objects on a plain sheet
• lattice.tst: menu operations on a subgroup lattice, including a regression test for SelectedGroups to GAP fails #26
• run.g: driver. XGAP shows GAP's output in its window and does not pass on its exit status, so results go to tst-gui.log and tst-gui.status; --quitonbreak and timeout keep an error from hanging the job.

Coverage

The run collects coverage for the GAP code (--cover) and the C code (gcov; build-pkg already compiles with --coverage), so the coverage upload is back. Codecov now shows 28 files, 49.78% overall: the 20 GAP files (e.g. lib/ilatgrp.gi, lib/gobject.gi) and the 8 C files (gapgraph.c 76%, xgap.c 55%, xcmds.c 43%, gaptext.c 26%, pty.c 25%, selfile.c 0%).

The C files only appeared after rebasing onto #47. With the old build, which compiled in bin/<arch>, codecov accepted the uploaded .gcov files without error but silently left all C files out of the report. An earlier version of this description said the ../../src.x11/ paths were not the cause; that was wrong, or at best incomplete. Rewriting the Source: line inside the .gcov files did not help, but compiling from the top level, which also changes the .gcov file names from ^#^#src.x11#xcmds.c.gcov to src.x11#xcmds.c.gcov, did.

codecov/project fails because codecov compares against a stored base of 93.71%, from when only the test driver was measured. It should settle once master has a report from this setup.

Assisted-by: Claude Code (Fable 5.1)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.78%. Comparing base (77840f6) to head (3564895).
⚠️ Report is 47 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (77840f6) and HEAD (3564895). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (77840f6)	HEAD (3564895)
	8	3

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #44       +/-   ##
===========================================
- Coverage   93.71%   49.78%   -43.93%     
===========================================
  Files           3       28       +25     
  Lines         175    10302    +10127     
  Branches        0      558      +558     
===========================================
+ Hits          164     5129     +4965     
- Misses         11     5083     +5072     
- Partials        0       90       +90     

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