[binutils-gdb] S390: Fix output path for s390-tdbregs test case

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 4a9321595003d86dd4794d0edab071f91989808e ***

Author: Andreas Arnez <arnez@linux.vnet.ibm.com>
Branch: master
Commit: 4a9321595003d86dd4794d0edab071f91989808e

S390: Fix output path for s390-tdbregs test case

Since test artifacts are always organized in a directory hierarchy, the
s390-tdbregs test case is not executed correctly any more.  This is
because it uses an obsolete way of constructing the executable's path.

This change invokes prepare_for_testing instead.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-tdbregs.exp: Use prepare_for_testing instead of
	manually constructing the output path.