testsuite: fix last "which" change

In commit afa63b2dcdc9b9 I replaced `type -p' with
`command -pv'.  But actually it is wrong, the right
substitution is `command -v'.  We need to find our
busybox which is in the first directory in $PATH, so
`command -p' should not be used because it uses
default PATH, not current value of PATH where our
busybox binary resides.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>