The regex to detect the beginning of a test file execution was too
broad, matching any line beginning with "Running ". This caused
dg-extract-results.sh to truncate some test logs, including
gdb.base/help.exp in the GDB test suite (which tests "help running",
printing "Running the program.")

The attached patch makes the match a little more precise to avoid
this. With this the exact count of PASS/FAIL/etc lines in the
dg-extract-results.sh output matches the counts in the computed
summary exactly for all cases I've seen.

I encountered this in the GDB test suite but was asked to submit the
change upstream to GCC as GDB would prefer to avoid unnecessary local
changes to the script & this change seems general enough to go in to
GCC.

I'm still a little fuzzy on the specifics of ChangeLog lines, but
here's my attempt at one for this change:

    * dg-extract-results.sh: constrain the start-of-log pattern to
avoid accidentally matching test content