The original logic of reducing a full path to a relative one assumes
"tests/misc" is in the filename which is limited in usage: it never works
for CPython on Windows since that will use a backslash as path separator,
and also won't work when the filename is a path not relative to the tests
directory which happens for example in the common case of running
"./run-tests -d misc".
Fix all cases by printing only the bare filename, which requires them all
to start with sys_settrace_ hence the renaming.
