Enable usage of llvm-lit on tests in test/ folder

[svuckovicTT]

Currently, it's not possible to run llvm-lit on any test in test/ folder. This is due to quirky nature of llvm-lit configs. Running llvm-lit within the build/test/ folder works however, as there's a lit.site.cfg.py config file that sets relevant paths for the tool to use.

This change enables usage of llvm-lit on test/ folder.

C̶M̶a̶k̶e̶'̶s̶ ̶̶c̶o̶n̶f̶i̶g̶u̶r̶e̶_̶f̶i̶l̶e̶̶ ̶m̶e̶t̶h̶o̶d̶ ̶i̶s̶ ̶u̶s̶e̶d̶ ̶t̶o̶ ̶p̶o̶p̶u̶l̶a̶t̶e̶ ̶"̶@̶.̶.̶.̶@̶"̶ ̶v̶a̶r̶i̶a̶b̶l̶e̶s̶,̶ ̶s̶u̶c̶h̶ ̶a̶s̶ ̶̶@̶L̶L̶V̶M̶_̶T̶O̶O̶L̶S̶_̶D̶I̶R̶@̶̶.̶ I̶t̶ ̶d̶o̶e̶s̶ ̶t̶h̶i̶s̶ ̶o̶n̶ ̶t̶h̶e̶ p̶r̶o̶v̶i̶d̶e̶d̶ ̶̶t̶e̶s̶t̶/̶l̶i̶t̶.̶s̶i̶t̶e̶.̶c̶f̶g̶.̶p̶y̶.̶i̶n̶̶ ̶f̶i̶l̶e̶,̶ ̶a̶n̶d̶ ̶o̶u̶t̶p̶u̶t̶s̶ ̶t̶h̶e̶ ̶r̶e̶s̶u̶l̶t̶ ̶t̶o̶ ̶̶t̶e̶s̶t̶/̶l̶i̶t̶.̶s̶i̶t̶e̶.̶c̶f̶g̶.̶p̶y̶̶ ̶(̶r̶e̶m̶o̶v̶e̶s̶ ̶t̶h̶e̶ ̶̶.̶i̶n̶̶ ̶e̶x̶t̶e̶n̶s̶i̶o̶n̶)̶.̶ T̶h̶e̶n̶,̶ ̶̶l̶l̶v̶m̶-̶l̶i̶t̶̶ ̶c̶a̶n̶ f̶i̶n̶d̶ ̶t̶h̶e̶ ̶c̶o̶n̶f̶i̶g̶ ̶f̶i̶l̶e̶ ̶a̶n̶d̶ ̶w̶o̶n̶'̶t̶ ̶c̶o̶m̶p̶l̶a̶i̶n̶ ̶a̶n̶y̶m̶o̶r̶e̶.̶

configure_lit_site_cfg is used to generate the config into test/ folder.

[svuckovicTT]

@mtopalovicTT I added you to the review as well, saw you did some stuff related to it :)

[nsmithtt]

This is awesome, thanks!