Fix an issue with a custom unit testing transport

platformio/builder/tools/piomisc.py

@@ -285,10 +285,12 @@ def ProcessTest(env):
         join("$BUILD_DIR", "UnityTestLib"), get_core_package_dir("tool-unity"))
     env.Prepend(LIBS=[unitylib])
 
-    src_filter = None
+    src_filter = [
+        "+<*.cpp>",
+        "+<*.c>"
+    ]
     if "PIOTEST" in env:
-        src_filter = "+<output_export.cpp>"
-        src_filter += " +<%s%s>" % (env['PIOTEST'], sep)
+        src_filter.append("+<%s%s>" % (env['PIOTEST'], sep))
 
     return env.CollectBuildFiles(
         "$BUILDTEST_DIR",