Fix for tagBam -names segfault

Allow overriding BT to test bedtools from other paths.

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}