Don't error if pytest skips all unit tests.

test/runner/lib/executor.py

@@ -515,7 +515,13 @@ def command_units(args):
 
     for version, command, env in version_commands:
         display.info('Unit test with Python %s' % version)
-        intercept_command(args, command, env=env, python_version=version)
+
+        try:
+            intercept_command(args, command, env=env, python_version=version)
+        except SubprocessError as ex:
+            # pytest exits with status code 5 when all tests are skipped, which isn't an error for our use case
+            if ex.status != 5:
+                raise
 
 
 def command_compile(args):