Add unit test for bam_split.c's filter_header_rg

Add unit test to test filter_header_rg function.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Restructure testing structure to dedupe

Move certain functions from test.h into a test.c file to reduce code
duplication.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>