Add trexe test
This adds an end-to-end test for trexe. It exercises handling of non-trivial command lines (arguments with spaces), stdout, input and output files.
An example of a successful run: https://gitlab.com/BuildGrid/trexe/-/jobs/2673864559
This depends on BuildGrid/trexe!9 (merged) and BuildGrid/trexe!8 (merged)