test: test-lib.sh: use $BASH instead of ${SHELL-sh} to relaunch

In case of the test script is to be relaunced under valgrind, or --tee
is requested, use the $BASH shell variable to locate the command
interpreter. The $SHELL variable is re-set by non-interactive shells
so in case the shell uses some other shell (e.g. zsh) for interactive
use these bash scripts continue to work.