Evaluate in the COMMAND arguments of custom commands the generator
expression syntax introduced in commit d2e1f2b4 (Introduce "generator
expressions" to add_test, 2009-08-11). These expressions have a syntax
like $<TARGET_FILE:mytarget> and are evaluated during build system
generation. This syntax allows per-configuration target output files to
be referenced in custom command lines.
Put the source files, build rules, and test scripts for these targets
under Tests/PerConfig and refer to it from Tests/Testing as a
subdirectory. The targets and scripts will be useful in other tests.