Speed up BCF2 and SeekableStream integration tests

[tfenne]

The test suite run time is dominated by a handful of classes whose combined method/test runtime are >> 1 minute each. I'm tackling the CRAM tests that offend, which is most of them, on my cram branch. But these two also stick out.

BCF2EncoderDecoderUnitTest (106s → <1s):
Reduce the forCombinations list from 17 to 9 values (one representative
per BCF2 type plus one null per non-CHAR type). This cuts the 3-way
cartesian product from 17^3=4913 to 9^3=729 test sequences while still
covering all type combinations and null handling. Total tests: 13848 → 1588.

SeekableStreamGZIPinputStreamIntegrationTest (82s → 4s):
Reduce from 1M to 100K VCF records per test file (sufficient to exercise
multi-block BGZF boundaries) and from 10 to 5 first-record-length
variants. Total tests: 20 → 10.

Full suite: 21,909 tests pass in 2m12s (was ~5m with 34,189 tests).

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