Improve diagnostics in TAsyncChannelTest
Improve diagnostics in TAsyncChannelTest

Summary:
Replace `CHECK_EQ` with `EXPECT_EQ` in `TAsyncChannelTest` to improve diagnostics on test failure. Without it the test will terminate on the first failed check such as
```
CHECK_EQ(sender_.error(), false);
```
without providing information what kind of error occurred.

Reviewed By: iahs

Differential Revision: D32247910

fbshipit-source-id: 94200bfba961f8b8e1caaf7de0d885eb828c90ad

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