Fix test_end2end.py (#369)

Summary:
Pull Request resolved: #369

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: 6b796834f2d77bfdd64372b0b6f47ee39a32cb5b

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",