Isolate build environment for "BuildSources" nodes

platformio/builder/tools/piomisc.py

@@ -306,9 +306,6 @@ def ProcessTest(env):
         src_filter.append("+<%s%s>" % (env['PIOTEST'], sep))
     env.Replace(PIOTEST_SRC_FILTER=src_filter)
 
-    return env.CollectBuildFiles("$BUILDTEST_DIR", "$PROJECTTEST_DIR",
-                                 "$PIOTEST_SRC_FILTER")
-
 
 def GetExtraScripts(env, scope):
     items = []