Modified the mpileup tests to have local copies of appropriate
reference sequences.  All other tests already had this (but
accidentally escaped in some circumstances).

Added a non-existant REF_PATH= to the tests to prevent it using the
default search path and escaping off to the EBI.

See https://github.com/samtools/samtools/issues/279
Replace bcftools+bcf_filter.sh with new vcf-miniview

Add new vcf-miniview test utility, which reads BCF or VCF and outputs
VCF optionally filtered in the same way that the old bcf_filter.sh did.
The sed -r option used by bcf_filter.sh was not available on OS X, and
an installation of bcftools will not necessarily be available on test
machines, notably when testing via travis.
Insulate test case from $GREP_OPTIONS

Prevent e.g. GREP_OPTIONS='--binary-files=without-match' from
subverting the expected output.

Perhaps we ought to add 'unalias egrep' somewhere too...
Update expected output with symbolic "<X>" alleles

Since b14b21933b08ccdf7a777ed4a3529edbb03114e9 mpileup outputs a
symbolic allele rather than a plain "X".
Update QS values in expected output, and don't truncate them

Following the lead of f85ac7d492ddf7fd54e9849733c2d66adf874938
(around the time of which the QS computation changed), we should be
regression testing the full values emitted for QS.
Merge minimal MD5 reference area for mpileup tests

Provide a local reference area via $REF_PATH for mpileup/regression.sh
tests (cf 38eda28b81fd3c176f90a0f03d2645fc891167f0's commit message).
(Note that these MD5 files are truncated versions of their full
sequences; just enough to pass the tests.)

Set REF_PATH to an empty value (but not the empty string, as that
produces the default setting), or in general to point within the test
suite, for tests that exercise it -- currently test.pl and regression.sh.

This makes the test suite self-contained for reference areas, and
in particular prevents travis build testing from trying to access
the EBI reference server.  Fixes #279.