[gdb/testsuite] Add missing wait in gdb.base/signals-state-child.exp

[gdb/testsuite] Add missing wait in gdb.base/signals-state-child.exp

On OBS I ran into:
...
(gdb) shell diff -s outputs/gdb.base/signals-state-child/standalone.txt \
  outputs/gdb.base/signals-state-child/gdb.txt^M
diff: outputs/gdb.base/signals-state-child/standalone.txt: \
  No such file or directory^M
(gdb) FAIL: gdb.base/signals-state-child.exp: signals states are identical
...

I managed to reproduce this by adding "sleep (5)" at the start of main in
signals-state-child.c.

Fix this by waiting on the result of the spawned command.

Tested on x86_64-linux.