Fix test_end2end.py

Summary:
A bunch of fixes including cleaning up imports, changing the style we
define test methods so that they can be discovered by unittest loader, fixing
small issues related to result comparing and disabling some tests that are not
easy to fix.

Reviewed By: Gasoonjia

Differential Revision: D49328465

fbshipit-source-id: 8c8a3e579d25e3af758d0c4f0034247aae712660

Author: larryliu0820

test/end2end/TARGETS

@@ -35,6 +35,7 @@ python_unittest(
     srcs = [
         "test_end2end.py",
     ],
+    preload_deps = ["//executorch/kernels/portable:custom_ops_generated_lib"],
     deps = [
         ":exported_module",
         ":register_scratch_meta_fns",
@@ -66,6 +67,7 @@ python_unittest(
     srcs = [
         "test_end2end.py",
     ],
+    preload_deps = ["//executorch/kernels/portable:custom_ops_generated_lib"],
     deps = [
         ":exported_module",
         ":register_scratch_meta_fns",