test-lib: add BUSYBOX prerequisite

When running with BusyBox, we will want to avoid calling executables on
the PATH that are implemented in BusyBox itself.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/test-lib.sh

@@ -1540,6 +1540,10 @@ test_lazy_prereq UNZIP '
 	test $? -ne 127
 '
 
+test_lazy_prereq BUSYBOX '
+	case "$($SHELL --help 2>&1)" in *BusyBox*) true;; *) false;; esac
+'
+
 run_with_limited_cmdline () {
 	(ulimit -s 128 && "$@")
 }