testsuite: Fix save-trace.exp writing outside standard output directory

testsuite: Fix save-trace.exp writing outside standard output directory

In save-trace.exp, we want to test loading of a tracepoint definition
file with a relative path (I am not sure why in fact).  We currently use
"savetrace-relative.tr", which ends up directly in testsuite/.  If we
use [standard_output_file] on that path, it becomes absolute.  I decided
to just replace [pwd] with . (a dot) in the path given by
standard_output_file to make it relative.  However, this trick only
works because [pwd] is a prefix of the standard output directory.  So I
added a check to verify that precondition.

gdb/testsuite/ChangeLog:

	* gdb.trace/save-trace.exp: Change relative path to be in the
	standard output directory.