Added 12 new tests to print header for files with large header from uncompressed files, gzip, and bgzip as file, redirect, pipe and fifo.

Added more unit tests for bed, gzip, and bgzipped files with large headers from file, redirect, pipe and fifo.

Fixed a bug in BamRecord::printUnmapped, added a unit test with with data file for unmapped Bam reads.

In order to test bedtools after it is installed in /usr/bin, it is needed
to override the BT variable in the shell scripts of the test suite.
The following change achieves it in compliance with POSIX (tested with dash).

-BT=../../bin/bedtools
+BT=${BT-../../bin/bedtools}