[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.

gdb/testsuite/gdb.server/connect-with-no-symbol-file.exp

@@ -75,9 +75,19 @@ proc connect_no_symbol_file { sysroot action } {
 
     # Check if GDB succeeded connecting to GDBserver by attempting to detach
     # from the process.
-    gdb_test "detach" \
-	".*Detaching from program: , process.*Ending remote debugging.*" \
-	"connection to GDBserver succeeded"
+    # Use an explicit fail clause to prevent the implicit clause matching
+    # "Ending remote debugging" from triggering, which would cause silent
+    # failure.
+    set ok 0
+    gdb_test_multiple "detach" "" {
+	-re "\r\nDetaching from \[^\r\n\]*" {
+	    set ok 1
+	    exp_continue
+	}
+	-re -wrap "" {
+	}
+    }
+    gdb_assert $ok "connection to GDBserver succeeded"
 }
 
 # Make sure we have the original symbol file in a safe place to copy from.