Refactor test to use do_runf and remove .out file

Author: subhaushsingh

test/jslib/test_jslib_custom_settings.out

@@ -353,13 +353,15 @@ def test_jslib_legacy(self):
   # the -jsDfoo=val syntax:
   # See https://github.com/emscripten-core/emscripten/issues/10580.
   def test_jslib_custom_settings(self):
-    self.cflags += ['--js-library', test_file('jslib/test_jslib_custom_settings.js'), '-jsDCUSTOM_JS_OPTION=1']
-    self.do_run_in_out_file_test('jslib/test_jslib_custom_settings.c')
-
-    self.run_process([EMCC, test_file('hello_world.c'), '-jsDCUSTOM_JS_OPTION=1', '-jsDCUSTOM_JS_OPTION=2'])
+    self.cflags += ['--js-library', test_file('jslib/test_jslib_custom_settings.js')]
 
     self.assert_fail([EMCC, '-jsDWASM=0'], 'cannot change built-in settings values with a -jsD directive')
 
+    self.do_runf('jslib/test_jslib_custom_settings.c', '1\n', cflags=['-jsDCUSTOM_JS_OPTION=1'])
+
+    # verify that the settings can be specified more than once, and that the last one wins.
+    self.do_runf('jslib/test_jslib_custom_settings.c', '2\n', cflags=['-jsDCUSTOM_JS_OPTION=1', '-jsDCUSTOM_JS_OPTION=2'])
+
   def test_jslib_native_deps(self):
     # Verify that memset (which lives in compiled code), can be specified as a JS library
     # dependency.