testsuite: Windows paths use \ and not /

Without this patch, the following error is reported on Windows:

In file included from t:\build\arm-none-eabi\include\c++\11.3.1\string:54,
                  from t:\build\arm-none-eabi\include\c++\11.3.1\bits\locale_classes.h:40,
                  from t:\build\arm-none-eabi\include\c++\11.3.1\bits\ios_base.h:41,
                  from t:\build\arm-none-eabi\include\c++\11.3.1\ios:42,
                  from t:\build\arm-none-eabi\include\c++\11.3.1\ostream:38,
                  from t:\build\arm-none-eabi\include\c++\11.3.1\iostream:39:
t:\build\arm-none-eabi\include\c++\11.3.1\bits\range_access.h:36:10: note: include 't:\build\arm-none-eabi\include\c++\11.3.1\initializer_list' translated to import
arm-none-eabi-g++.exe: warning: .../gcc/testsuite/g++.dg/modules/pr99023_b.X: linker input file unused because linking not done
FAIL: g++.dg/modules/pr99023_b.X -std=c++2a  dg-regexp 6 not found: "[^\n]*: note: include '[^\n]*/initializer_list' translated to import\n"

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr99023_b.X: Match Windows paths too.

Co-Authored-By: Yvan ROUX <yvan.roux@foss.st.com>
Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>