Put selftests api into selftests namespace
    
    This patch changes register_self_test to selftests::register_test,
    and run_self_tests to selftest::run_tests.
    
    gdb:
    
    2017-08-18  Yao Qi  <yao.qi@linaro.org>
    
    	* selftest.c (register_self_test): Rename it to
    	selftests::register_test.
    	(run_self_tests): selftest::run_tests.
    	* selftest.h: Update declarations.
    	* selftest-arch.c (register_self_test_foreach_arch): Rename it to
    	selftests::register_test_foreach_arch.
    	* selftest-arch.h: Update declaration.
    	* aarch64-tdep.c: Update.
    	* arm-tdep.c: Likewise.
    	* disasm-selftests.c: Likewise.
    	* dwarf2loc.c: Likewise.
    	* dwarf2-frame.c: Likewise.
    	* findvar.c: Likewise.
    	* gdbarch-selftests.c: Likewise.
    	* maint.c (maintenance_selftest): Likewise.
    	* regcache.c: Likewise.
    	* rust-exp.y: Likewise.
    	* selftest-arch.c: Likewise.
    	* unittests/environ-selftests.c: Likewise.
    	* unittests/function-view-selftests.c: Likewise.
    	* unittests/offset-type-selftests.c: Likewise.
    	* unittests/optional-selftests.c: Likewise.
    	* unittests/scoped_restore-selftests.c: Likewise.
    	* utils-selftests.c: Likewise.