gdb: fix failures in gdb.mi/mi-exec-run.exp with native-extended-gdbserver

gdb: fix failures in gdb.mi/mi-exec-run.exp with native-extended-gdbserver

When running the gdb.mi/mi-exec-run.exp test using the
native-extended-gdbserver I see failures like this:

  FAIL: gdb.mi/mi-exec-run.exp: inferior-tty=main: mi=main: force-fail=1: run failure detected
  FAIL: gdb.mi/mi-exec-run.exp: inferior-tty=main: mi=separate: force-fail=1: run failure detected
  FAIL: gdb.mi/mi-exec-run.exp: inferior-tty=separate: mi=separate: force-fail=1: run failure detected

There's a race condition here, so you might see a slightly different
set of failures, but I always see some from the 'run failure detected'
test.

NOTE: I also see an additional test failure:

 FAIL: gdb.mi/mi-exec-run.exp: inferior-tty=separate: mi=separate: force-fail=0: breakpoint hit reported on console (timeout)

but that is a completely different issue, and is not being addressed
in this commit.

The problem for the 'run failure detected' test is that we end up
in gdb_expect looking for output from two spawn-ids, one from
gdbserver, and one from gdb.  We're looking for one output pattern
from each spawn-id, and for the test to pass we need to see both
patterns.

Now, if gdb exits then this is a test failure (this would indicate gdb
crashing, which is bad), so we have an eof pattern associated with
the gdb spawn-id.

However, in this particular test we expect gdbserver to fail to
execute the binary (the test binary is set non-executable), and so we
get an error message from gdbserver (which matches the pattern), and
then gdbserver exits, this is expected.

The problem is that after spotting the pattern from gdbserver, we
often see the eof from gdbserver before we see the pattern from gdb.
If this happens then we drop out of the gdb_expect without ever seeing
the pattern from gdb, and fail the test.

In this commit, I place the spawn-id of gdbserver into a global
variable, and then use this global variable as the -i option within
the gdb_expect.

Now, once we have seen the expected pattern on the gdbserver spawn-id,
the global variable is cleared.  After this the gdb_expect no longer
checks the gdbserver spawn-id for additional output, and so never sees
the eof event.  This leaves the gdb_expect running, which allows the
pattern from gdb to be seen, and for the test to pass.

I now see no failures relating to 'run failure detected'.