[testsuite] Don't use 'testfile' before 'standard_testfile'.

[testsuite] Don't use 'testfile' before 'standard_testfile'.

While running tests on arm-none-eabi, I noticed following errors in some
gdb.threads tests.

ERROR: can't read "testfile": no such variable

These were being caused by ${testfile} being used before 'standard_testfile'
which sets it. This patch just moves standard_testfile before the use.

2021-02-09  Abid Qadeer  <abidh@codesourcery.com>

	gdb/testsuite/ChangeLog:

	* gdb.threads/signal-command-handle-nopass.exp: Call
	'standard_testfile' before using 'testfile'.
	* gdb.threads/signal-command-multiple-signals-pending.exp: Likewise.
	* gdb.threads/signal-delivered-right-thread.exp: Likewise
	* gdb.threads/signal-sigtrap.exp: Likewise