Fix file upload tests

We can't easily select multiple files in acceptance tests, so don't try that.

test/acceptance/pages.py

@@ -178,7 +178,7 @@ class SubmissionPage(OpenAssessmentPage):
         Returns:
             bool
         """
-        return self.q(css="button.file__upload").attrs('disabled') == ['false']
+        return self.q(css="button.file__upload")[0].is_enabled()
 
     @property
     def upload_file_button_is_disabled(self):

test/acceptance/tests.py

@@ -741,18 +741,12 @@ class FileUploadTest(OpenAssessmentTest):
         self.assertTrue(self.submission_page.has_file_error)
 
         # trying to upload a acceptable file
-        readme1 = os.path.dirname(os.path.realpath(__file__)) + '/README.rst'
-        readme2 = readme1.replace('test/acceptance/', '')  # There's another README located at ../../
-
-        files = ', '.join([readme1, readme2])
-        self.submission_page.visit().select_file(files)
+        readme = os.path.dirname(os.path.realpath(__file__)) + '/README.rst'
+        self.submission_page.visit().select_file(readme)
         self.assertFalse(self.submission_page.has_file_error)
         self.assertTrue(self.submission_page.upload_file_button_is_disabled)
 
         self.submission_page.add_file_description(0, 'file description 1')
-        self.assertTrue(self.submission_page.upload_file_button_is_disabled)
-
-        self.submission_page.add_file_description(1, 'file description 2')
         self.assertTrue(self.submission_page.upload_file_button_is_enabled)
 
         self.submission_page.upload_file()