gdb.base/fileio.c: Fix several -Wreturn-type warnings

gdb.base/fileio.c: Fix several -Wreturn-type warnings

All the "test_" functions warn like:

  src/gdb/testsuite/gdb.base/fileio.c: In function ‘test_close’:
  src/gdb/testsuite/gdb.base/fileio.c:280:1: warning: control reaches end of non-void function [-Wreturn-type]
   }
   ^

Nothing looks at the return of these functions, so just make them
return void.  While at it, "()" is not the same as "(void)" in C - fix
that too.

gdb/ChangeLog:
2017-05-18  Pedro Alves  <palves@redhat.com>

	* gdb.base/fileio.c (stop, test_open, test_write, test_read)
	(test_lseek, test_close, test_stat, test_fstat, test_isatty)
	(test_system, test_rename, test_unlink, test_time): Change
	prototypes.
	* gdb.base/fileio.exp (stop_msg): Adjust.