FFmpeg: Add regression tests

Currently only covering handful of files from reports about wrong fps detected.

It will need D3083 applied first to get tests passed, also tests themselves
are to be committed to svn.

But there are some python code which needs to be reviewed, like blendfile
passed to run_blender().

Reviewers: sybren, mont29

Reviewed By: sybren, mont29

Subscribers: mont29

Differential Revision: https://developer.blender.org/D3096

tests/python/modules/test_utils.py

@@ -75,18 +75,24 @@ class AbstractBlenderRunnerTest(unittest.TestCase):
         assert self.blender, "Path to Blender binary is to be set in setUpClass()"
         assert self.testdir, "Path to tests binary is to be set in setUpClass()"
 
-        blendfile = self.testdir / filepath
+        blendfile = self.testdir / filepath if filepath else ""
 
-        command = (
+        command = [
             self.blender,
             '--background',
             '-noaudio',
             '--factory-startup',
             '--enable-autoexec',
-            str(blendfile),
+        ]
+
+        if blendfile:
+            command.append(str(blendfile))
+
+        command.extend([
             '-E', 'CYCLES',
             '--python-exit-code', '47',
             '--python-expr', python_script,
+            ]
         )
 
         proc = subprocess.run(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,