Unit test for writeBambuOutput and update bambu output

[lingminhao]

1. In this pull request, I added some unit tests for the writeBambuOutput() function. It tests whether:

• the prefix is as expected
• the output files go to the correct output path
• the file extensions are correct
• the column names for the count outputs include "GENEID" and/or "TXNAME".
• the sample columns for count output files are numeric.

2. Additionally, since the theta column was removed from the Bambu output, some of the internal/external data have been updated.

3. The objects used for generic testing for quantification have also been updated.