Fix symbolicate tests stack trace diff on Windows

Fix symbolicate tests stack trace diff on Windows

The symbolicate tests compare stack traces generated by the mono-symbolicate
tool against expected stack traces. The expected stack trace files have been
preprocessed to remove absolute paths and the Makefile does the same with the
output from the tool. However, the preprocessing doesn't take different
directory separator characters into account. This patch adds a 'tr' command to
the Makefile which converts \ -> / before the absolute paths are filtered out
from the actual output from the tool to make the comparison succeed on Windows
as well.