Use do_self_tests in selftest.exp

This patch uses do_self_tests to simplify selftest.exp.  It doesn't
change the tests except the order,

-PASS: gdb.gdb/selftest.exp: Disassemble main
 PASS: gdb.gdb/selftest.exp: breakpoint in captured_main
+PASS: gdb.gdb/selftest.exp: run until breakpoint at captured_main
+PASS: gdb.gdb/selftest.exp: Disassemble main
 PASS: gdb.gdb/selftest.exp: set interrupt character in test_with_self
 PASS: gdb.gdb/selftest.exp: set listsize to 1
-PASS: gdb.gdb/selftest.exp: run until breakpoint at captured_main

gdb/testsuite:

2016-07-19  Yao Qi  <yao.qi@linaro.org>

	* gdb.gdb/selftest.exp: Remove checks on is_remote and isnative.
	(test_with_self): Remove some code.  Remove argument executable.
	(top-level): Use do_self_tests.