build: make check: use a PATH search instead for Kyua

which(1) accepts both relative/absolute paths as well as lone binary
names. Set KYUA to kyua and use which(1) to confirm that it can find one;
if it cannot, just advise the user to set KYUA directly to the kyua binary
rather than assuming a relative location from LOCALBASE.

This allows make check to be operated with the version of kyua in base
without losing the flexibility of specifying another one.

ngie@ notes that the original intention was to avoid redundant $PATH lookups
and improve the determinism of the target. A future change will likely push
us back to this state, perhaps in the form of reverting this entirely and
just switching to using kyua in base. Accepting any in $PATH should be
considered a transitional move, at least until it's declared otherwise,
since kyua was only semi-recently added to base.

Reviewed-by: brooks, emaste, lwhsu, ngie
Differential-Revision: https://reviews.freebsd.org/D28045