gdb/testsuite: make gdb_test_multiple return immediately if send_gdb fails

gdb/testsuite: make gdb_test_multiple return immediately if send_gdb fails

In the failure seen by Philippe here:

  https://inbox.sourceware.org/gdb-patches/20221120173024.3647464-1-philippe.waroquiers@skynet.be/

... the testsuite only outputs PASSes, and an ERROR, resulting from an
uncaught exception.  This is a bit sneaky, because ERRORs are not
reported in the test summary.  In certain circumstances, it can be easy
to miss.

Normally, gdb_test_multiple outputs an UNRESOLVED when GDB crashes.  But
this is only if it manages to send the command, and it's that command
that crashes GDB.  Here, the ERROR is due to the fact that GDB had
already crashed by the time we entered gdb_test_multiple and tried to
send a command.  GDB was crashed by the previous "file" command, sent by
gdb_unload.  Because gdb_unload uses bare expect, it didn't record a
test failure when crashing GDB (this will be addressed separately).

In this patch, I propose to make gdb_test_multiple call unresolved
directly and return -1 send_gdb fails.  This way, if GDB is already
crashed by the time we enter gdb_test_multiple, it will leave a trace in
the test results in the form of an UNRESOLVED.  It will also spare us
the not-so-useful-in-my-opinion TCL backtrace.

Before, it looks like:

    ERROR: Couldn't send python print(objfile.filename) to GDB.
    ERROR: : spawn id exp9 not open
        while executing
    "expect {
    -i exp9 -timeout 10
            -re ".*A problem internal to GDB has been detected" {
                fail "$message (GDB internal error)"
                gdb_internal_error..."
        ("uplevel" body line 1)
        invoked from within
    "uplevel $body" NONE : spawn id exp9 not open

And after:

    Couldn't send python print(objfile.filename) to GDB.
    UNRESOLVED: gdb.python/py-objfile.exp: objfile.filename after objfile is unloaded

Change-Id: I72af8dc0d687826fc3f76911c27a9e5f91b677ba
Approved-By: Tom de Vries <tdevries@suse.de>