Testsuite: Catch gdbserver socket listen errors

Testsuite: Catch gdbserver socket listen errors

When launching gdbserver, the testsuite checks for binding failure but
does not check for failure to listen to socket error (which can happen
due to another gdbserver binding to the socket at the same time).

When this error occurs, the test will ignore the error and connect GDB
to the failed port.  This may succeed and GDB will now be connected to
the gdbserver from another test.  This eventually causes both tests to
fail.

When running the tests suite with native-gdbserver across many cores,
this issue may happen once or twice, each causing random failures for
two .exp testscripts.

Example gdb.log output for the failure:

The testsuite sucessfully notices a failure to connect to port 2348.
It launches again with port 2349, which also fails.  The testsuite
ignores this error and uses gdb to connect to the port - which succeeds.

spawn /work/build/gdb/testsuite/../gdbserver/gdbserver --once localhost:2348 /work/build/gdb/testsuite/outputs/gdb.ada/arrayidx/p^M
Can't bind address: Address already in use.^M
Exiting^M
Port 2348 is already in use.
spawn /work/build/gdb/testsuite/../gdbserver/gdbserver --once localhost:2349 /work/build/gdb/testsuite/outputs/gdb.ada/arrayidx/p^M
Can't listen on socket: Address already in use.^M
Exiting^M
target remote localhost:2349^M
Remote debugging using localhost:2349^M
Reading /lib/ld-linux-aarch64.so.1 from remote target...^M
warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.^M
Reading /lib/ld-linux-aarch64.so.1 from remote target...^M
Reading symbols from target:/lib/ld-linux-aarch64.so.1...^M
Reading /lib/ld-2.23.so from remote target...^M
Reading /lib/.debug/ld-2.23.so from remote target...^M
Reading /work/build/install/lib/debug//lib/ld-2.23.so from remote target...^M
Reading /work/build/install/lib/debug/lib//ld-2.23.so from remote target...^M
Reading target:/work/build/install/lib/debug/lib//ld-2.23.so from remote target...^M
(No debugging symbols found in target:/lib/ld-linux-aarch64.so.1)^M
0x0000ffffbf6d2cc0 in ?? () from target:/lib/ld-linux-aarch64.so.1^M
(gdb) continue^M
Continuing.^M
Reading /lib/aarch64-linux-gnu/libc.so.6 from remote target...^M
Reading /lib/aarch64-linux-gnu/libc-2.23.so from remote target...^M
Reading /lib/aarch64-linux-gnu/.debug/libc-2.23.so from remote target...^M
Reading /work/build/install/lib/debug//lib/aarch64-linux-gnu/libc-2.23.so from remote target...^M
Reading /work/build/install/lib/debug/lib/aarch64-linux-gnu//libc-2.23.so from remote target...^M
Reading target:/work/build/install/lib/debug/lib/aarch64-linux-gnu//libc-2.23.so from remote target...^M
[Inferior 1 (process 35351) exited normally]^M
(gdb) FAIL: gdb.ada/arrayidx.exp: can't run to main

Meanwhile, at the same time, in another test, gdbserver successfully
connects to port 2349.  GDB then tries to connect to the port, but it
times out because the GDB in the test above has already connected to it.

spawn /work/build/gdb/testsuite/../gdbserver/gdbserver --once localhost:2348 /work/build/gdb/testsuite/outputs/gdb.ada/rdv_wait/foo^M
Can't bind address: Address already in use.^M
Exiting^M
Port 2348 is already in use.
spawn /work/build/gdb/testsuite/../gdbserver/gdbserver --once localhost:2349 /work/build/gdb/testsuite/outputs/gdb.ada/rdv_wait/foo^M
Process /work/build/gdb/testsuite/outputs/gdb.ada/rdv_wait/foo created; pid = 65162^M
Listening on port 2349^M
Remote debugging from host 127.0.0.1, port 45154^M
target remote localhost:2349^M
localhost:2349: Connection timed out.^M
(gdb) ^CQuit^M
(gdb) task 2^M
Cannot inspect Ada tasks when program is not running^M

gdb/testsuite/ChangeLog:

	* lib/gdbserver-support.exp (gdbserver_start): Check for listen
	failure.