[gdb/testsuite] Fix silent fail in gdb.server/connect-with-no-symbol-file.exp

[gdb/testsuite] Fix silent fail in gdb.server/connect-with-no-symbol-file.exp

With native and target boards native-gdbserver, remote-gdbserver-on-localhost and
remote-stdio-gdbserver I have for gdb.server/connect-with-no-symbol-file.exp:
...
 # of expected passes            8
...
but with native-extended-gdbserver I have instead:
...
 # of expected passes            8
 # of unexpected failures        4
...

The extra FAILs are of the form:
...
(gdb) detach^M
Detaching from pid process 28985^M
[Inferior 1 (process 28985) detached]^M
(gdb) FAIL: gdb.server/connect-with-no-symbol-file.exp: sysroot=: \
  action=permission: connection to GDBserver succeeded
...
and are due to the fact that the actual gdb output doesn't match the regexp:
...
    gdb_test "detach" \
       ".*Detaching from program: , process.*Ending remote debugging.*" \
       "connection to GDBserver succeeded"
...

With native, the actual gdb output is:
...
(gdb) detach^M
Detaching from pid process 29657^M
Ending remote debugging.^M
[Inferior 1 (process 29657) detached]^M
(gdb) Remote debugging from host ::1, port 51028^M
...
and because the regexp doesn't match, it triggers an implicit clause for
"Ending remote debugging" in gdb_test_multiple, which has the consequence
that the FAIL is silent.

Fix:
- the regexp by making it less strict
- the silent fail by rewriting into a gdb_test_multiple, and adding an
  explicit fail clause.

Tested on x86_64-linux, using native and aforementioned target boards.