fix flaky test in TracerTest.java

[zcai2]

Fixed flaky test that caused by setExceptionPredicate.

For setExceptionsToTrace and setExceptionsToIgnore, if we called setExceptionPredicate first, it will assigned value to Tracer.exceptionPredicate.

To avoid this case, we would better to reset exceptionPredicate to null first

The flaky tests are found by running https://github.com/idflakies/iDFlakies

[CLAassistant]

All committers have signed the CLA.

[sczyh30]

Hi, thanks for contributing. Could you please sign the CLA here? And please make sure the email of your commits match your GitHub email. You may refer to the instruction here, or rebase your commits with the correct email.

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感谢贡献，请将 commit 对应的 email 调整成与 GitHub 的 email 相匹配并 确认一下 CLA。

[zcai2]

Hi,
I have signed the CLA and update my email as well.

[sczyh30]

It could be better if you could clear the exceptionPredicate via @Before and @After period.

[sczyh30]

Thanks for contributing!