Modifies test.pl so that it exits with non-zero status if any tests fail

I noticed when I put in the last pull request for the usage tests (which are failing for 4 subcommands) that the travis build did not fail as expected.  This brings samtools' test.pl in line with bcftools such that it exits with a status equal to the number of tests that failed (seems reasonable up to a point -- possibly might be better to just use 0/1 but we should probably be consistent across samtools/bcftools).