Remove some GDB specific stuff from selftest.c

Remove some GDB specific stuff from selftest.c

The next patch moves selftest.c to common/selftest.c, so that GDBserver
can use it as well.  However selftest.c uses something isn't "portable" on
GDB and GDBserver.

First, this patch removes QUIT.  I don't expect that we type ctrl-c during
self/unit tests, and each test shouldn't take long time.  Secondly, I
replace exception_fprintf and printf_filtered with debug_printf.  Verified
that unit tests still catch fails.

gdb:

2017-08-18  Yao Qi  <yao.qi@linaro.org>

	* selftest.c (run_tests): Don't call QUIT.  Call debug_printf
	instead of exception_fprintf and printf_filtered.