GHA: Some improvements to the GHA environemnt

[k-takata]

Extracted from #6795.

• Fix that some tests were not executed on GHA.
  To fix this, copy all the files in src/ (not only src/testdir/) to src2/, and fix some tests.
  (The tests for gvim.exe is run at src/testdir/, and the tests for vim.exe is run at src2/testdir/.)
• Update the flaky tests list.
  Some tests are flaky on GHA. Test_BufWrite_lockmarks() and Test_bufunload_all() are added to the list.
  Actually, there are some more flaky tests on GHA, but at least these two tests are often failed.
  (I tried to fix them, but I couldn't. They don't reproduce in my local environment.)

[codecov]

Codecov Report

Merging #6798 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6798      +/-   ##
==========================================
- Coverage   88.51%   88.51%   -0.01%     
==========================================
  Files         147      147              
  Lines      160891   160891              
==========================================
- Hits       142414   142411       -3     
- Misses      18477    18480       +3     

Impacted Files	Coverage Δ
src/netbeans.c	76.07% <0.00%> (-0.23%)	⬇️
src/gui.c	63.30% <0.00%> (-0.15%)	⬇️
src/message.c	88.48% <0.00%> (-0.05%)	⬇️
src/terminal.c	90.98% <0.00%> (-0.04%)	⬇️
src/gui_gtk_x11.c	58.73% <0.00%> (ø)
src/window.c	92.50% <0.00%> (+0.03%)	⬆️
src/channel.c	89.83% <0.00%> (+0.03%)	⬆️
src/clipboard.c	83.31% <0.00%> (+0.10%)	⬆️
src/if_xcmdsrv.c	88.90% <0.00%> (+0.35%)	⬆️

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[k-takata]

@brammool I think this is worth to be merged.
This fixes some issues with current tests on GHA.

• Fix that some tests were not executed on GHA.

We have more than 3,000 tests, but only about 1,600 tests were executed on the console version on GHA.