Some l3build configs are not used on CI

[muzimuzhi]

I find some l3build configs are not used on CI, thus the tests are not checked by CI.

Among them, base/config-lttemplates.lua will be added if PR #1534 gets merged. The remaining are

# not added to "checkconfigs" in corresponding build.lua, and
# not covered by job matrixes in main.yaml and pretest.yaml
base/config-broken.lua
base/config-filenames.lua
base/config-search.lua
required/latex-lab/config-OR-local.lua
required/latex-lab/config-TU.lua
required/latex-lab/config-math-tagging-examples.lua
required/tools/config-broken.lua

# not added to "checkconfigs" in required/firstaid/build.lua
required/firstaid/config-local.lua
required/firstaid/config-pdf.lua

I guess the two config-broken.lua should not be checked, as suggested by there names.

Note that base/config-filenames.lua sets testfiledir = "testfiles-filenames" but the corresponding directory base/testfiles-filenames doesn't exist. There's only base/testfiles-filename (without the trailing "s").

[josephwright]

I've merged #1534

[josephwright]

Sorted?

[muzimuzhi]

If any configs listed in the code block in #1536 (comment) need to be tested, then no, not sorted.