bootstrap: fix failing diagnostic selftest on Windows [PR98696]

In one of the selftests in g:f1096055
I didn't consider that paths can contain backslashes, which happens
for the tempfiles on Windows hosts.

gcc/ChangeLog:
	PR bootstrap/98696
	* diagnostic.c
	(selftest::test_print_parseable_fixits_bytes_vs_display_columns):
	Escape the tempfile name when constructing the expected output.