catch_discover_tests needs to support relative paths

**Description**
catch_discover_tests needs to support relative paths. currently it populates _include.cmake files with absolute paths for DISCOVERY_MODE PRE_TEST.
Making them relative allows user to seperate the build and execution on different platforms/systems. This allows portability of test artifacts.

**Additional context**
https://github.com/catchorg/Catch2/blob/devel/extras/Catch.cmake#L263 -
TEST_EXECUTABLE" " [==[" "$" "]==]" "\n"

$ populates the absolute path during the build time. Which does not exist at execution runtime

Need to create a flag USE_RELATIVE_PATHS to allow user to pass relative paths instead of having them populated at build time

Start in extras/Catch.cmake around the TEST_EXECUTABLE entry for DISCOVERY_MODE PRE_TEST. Trace how TARGET_FILE is used when generating _include.cmake files, then determine how the requested USE_RELATIVE_PATHS option should select relative paths. Done means PRE_TEST discovery can produce portable relative paths instead of build-time absolute paths.

Written by the indexing model from the issue text.