Make `_test_xla_generate` less flaky
# What does this PR do?

Make `_test_xla_generate` less flaky by relaxing the condition:
- if number of examples < 10: be strict, no difference is allowed
- otherwise, only fail the test if there are more than 10% of examples give different outputs between XLA and non-XLA versions.

Since this test is slow (generation), better not to decorate with `is_flaky`.

For `TFPegasusModelTest::test_xla_generate_slow`: there were more than 10 failures in 70 runs. With this PR, 0 failure shows.

_The documentation is not available anymore as the PR was closed or merged._