Minor update to the dev_fix_debug_* test-programs to fix bit-rot issues.

The test-programs in cv32e40p/tests/programs/custom/debug_test_* were no longer compiling due to:

1. Missing #include <stdint.h>
2. Null handle for uvmt_cv32e40p_test_randvars_c::test_randvars

This PR resolves both issues, and adds a minor cleanup of cv32e40p/sim/tools/xrun/README.md.