[test] Pass filename explicitly to compile_btest helper. NFC

This allows us to use `compiler_for` to determine whether to run
`EMCC` or `EMXX` appropriately.

Author: sbc100

test/common.py

@@ -2106,7 +2106,7 @@ def reftest(self, expected, manually_trigger=False):
       setupRefTest();
 ''' % (reporting, basename, int(manually_trigger)))
 
-  def compile_btest(self, args, reporting=Reporting.FULL):
+  def compile_btest(self, filename, args, reporting=Reporting.FULL):
     # Inject support code for reporting results. This adds an include a header so testcases can
     # use REPORT_RESULT, and also adds a cpp file to be compiled alongside the testcase, which
     # contains the implementation of REPORT_RESULT (we can't just include that implementation in
@@ -2123,7 +2123,9 @@ def compile_btest(self, args, reporting=Reporting.FULL):
                  test_file('report_result.c')]
     if EMTEST_BROWSER == 'node':
       args.append('-DEMTEST_NODE')
-    self.run_process([EMCC] + self.get_emcc_args() + args)
+    if not os.path.exists(filename):
+      filename = test_file(filename)
+    self.run_process([compiler_for(filename), filename] + self.get_emcc_args() + args)
 
   def btest_exit(self, filename, assert_returncode=0, *args, **kwargs):
     """Special case of btest that reports its result solely via exiting
@@ -2166,10 +2168,10 @@ def btest(self, filename, expected=None, reference=None,
       # manual_reference only makes sense for reference tests
       assert manual_reference is None
     outfile = output_basename + '.html'
-    args += [filename, '-o', outfile]
+    args += ['-o', outfile]
     # print('all args:', args)
     utils.delete_file(outfile)
-    self.compile_btest(args, reporting=reporting)
+    self.compile_btest(filename, args, reporting=reporting)
     self.assertExists(outfile)
     if post_build:
       post_build()