gdb.python/py-prompt.exp: restore GDBFLAGS

gdb.python/py-prompt.exp: restore GDBFLAGS

The previous change to py-prompt.exp made it return without restoring
GDBFLAGS, resulting in breaking the following tests:

  $ make check RUNTESTFLAGS="--target_board=native-gdbserver --directory=gdb.python"
  ...
  Running src/gdb/testsuite/gdb.python/py-prompt.exp ...
  Running src/gdb/testsuite/gdb.python/py-section-script.exp ...
  ERROR: (timeout) GDB never initialized after 10 seconds.
  ERROR: no fileid for gdbuild
  ERROR: Couldn't send python print ('test') to GDB.
  ERROR: no fileid for gdbuild
  ERROR: Couldn't send python print (sys.version_info[0]) to GDB.
  ERROR: no fileid for gdbuild
  ERROR: Couldn't send python print (sys.version_info[1]) to GDB.
  ERROR: no fileid for gdbuild
  ERROR: no fileid for gdbuild
  ...

gdb/testsuite/
2015-01-12  Pedro Alves  <palves@redhat.com>

	* gdb.python/py-prompt.exp: When the board can't spawn for attach,
	restore GDBFLAGS before returning.