Mpileup test

Added some tests for mpileup.

Note that this needs bcftools in order to check the BCF output (I can't grok the binary stream and remove version identifiers easily without converting to text form first), and so it has an explicit assumption of ../bcftools subdirectory along side the samtools directory.

This is still my bourne-shell test setup, although I updated it to include expected failures. The main thing is the test commands and expected outputs though; changing to whatever our favourite test harness is (we already have 2 homebrew solutions, 3 with sh) is left as an exercise to the reader. :-)