Refactor Bzip2 tests

Motivation:

Complicated code of Bzip2 tests with some unnecessary actions.

Modifications:

- Reduce size of BYTES_LARGE array of random test data for Bzip2  tests.
- Removed unnecessary creations of EmbeddedChannel instances in Bzip2 tests.
- Simplified tests in Bzip2DecoderTest which expect exception.
- Removed unnecessary testStreamInitialization() from Bzip2EncoderTest.

Result:

Reduced time to test the 'codec' package by 7 percent, simplified code of Bzip2 tests.