gdb/testsuite: mark the start of each gdb.in.* log file

gdb/testsuite: mark the start of each gdb.in.* log file

Emit a line in the gdb.log file each time a new gdb.in.NUM command log
is started.  The gdb.log line includes the full filename for the new
gdb.in.NUM file.

This change will make it trivial to go from a FAIL in the gdb.log file
to the gdb.in.NUM file that (should) reproduce the failure.  When I
encounter a failing test one of my first steps is usually to identify
the gdb.in.NUM file and try re-running it to see if that reproduces
the failure.  Some tests create many very similar gdb.in.NUM files, so
finding the exact one can sometimes be difficult.  With this patch
that task is now trivial.

There should be no change in what is tested after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>