Disable fixmate test (for now) due to behaviour change

From the SAMtools v1.21 release notes: `samtools fixmate` no longer
removes the PAIRED flag from reads that have no mate. This is done
on the understanding that the PAIRED flag is a sequencing technology
indicator not a feature of alignment.

tests/samtools_test.py

@@ -83,7 +83,8 @@ class SamtoolsTest(unittest.TestCase):
  "index ex1.bam %(out)s_ex1.bam.fai",
  "index -@2 ex1.bam %(out)s_ex1.bam.fai",
  "idxstats ex1.bam > %(out)s_ex1.idxstats",
- "fixmate ex1.bam %(out)s_ex1.fixmate.bam",
+ # TODO: fixmate behaviour changed in 1.21
+ #"fixmate ex1.bam %(out)s_ex1.fixmate.bam",
  "flagstat ex1.bam > %(out)s_ex1.flagstat",
  "calmd ex1.bam ex1.fa > %(out)s_ex1.calmd.bam",
  # use -s option, otherwise the following error in samtools 1.2: