ztest: take -B ./path/to/ztest, LD_LIBRARY_PATH=./path/lib:$L_L_P

This changes the behaviour of -B from the illumos one which would,
in the example in the manual, take just ./chroots/lenny;
this, however, is more versatile, and scales much better for systems
with ZFS in /usr/local, for example

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13411
Closes #1770