Fix model test timeout issue

Summary:
As I'm working on adding more complex models to the example, I often run into timeout issue when running test even for a single model. This diff to to mark model e2e test 'long_running' *(10mins if w/o 'long_running' flag)*. Now both MV2 and MV3 are disabled by the CI due to timeout (https://fburl.com/test/2xz69n58).

Shortly we should make one test target per model to fully remove dependencies between models the maximize the throughput of the CI.

Reviewed By: chakriu

Differential Revision: D48133248

fbshipit-source-id: 51ff99bd5fbf556da6c55bb3f2f201da407ac93a

Author: guangy10

examples/export/test/TARGETS

@@ -5,6 +5,7 @@ python_unittest(
     srcs = [
         "test_export.py",
     ],
+    tags = ["long_running"],
     deps = [
         "//caffe2:torch",
         "//executorch/examples/export:utils",