tp: Refactor diff_test script to standalone library

Bug:255535171
Change-Id: Ib4f12b7034feb9c8aa4fd183816ffc78a4cfd402

python/BUILD.gn

diff --git a/python/BUILD.gn b/python/BUILD.gn
index 6dcf9ed..06ebd9c 100644
--- a/python/BUILD.gn
+++ b/python/BUILD.gn
@@ -31,6 +31,14 @@
   ]
 }
 
+perfetto_py_library("trace_processor_diff_tests") {
+  sources = [
+    "generators/diff_tests/runner.py",
+    "generators/diff_tests/testing.py",
+    "generators/diff_tests/utils.py",
+  ]
+}
+
 perfetto_py_binary("trace_processor_py_example") {
   sources = [ "example.py" ]
   deps = [