Fix the tests

[haifeng-jin]

• make tests faster
• skip tests that always fail on tf-nightly

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (5abd2d5) 100.00% compared to head (85a03b6) 100.00%. Report is 2 commits behind head on master.

:exclamation: Current head 85a03b6 differs from pull request most recent head 817a12f. Consider uploading reports for the commit 817a12f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1894 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 50 50 Lines 2927 2927 ========================================= Hits 2927 2927 ``` | [Files](https://app.codecov.io/gh/keras-team/autokeras/pull/1894?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | Coverage Δ | | |---|---|---| | [autokeras/tasks/structured\_data.py](https://app.codecov.io/gh/keras-team/autokeras/pull/1894?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team#diff-YXV0b2tlcmFzL3Rhc2tzL3N0cnVjdHVyZWRfZGF0YS5weQ==) | `100.00% <ø> (ø)` | |

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