commit to add scp to the test sshd's path causes the t-envpass test to fail
when the test scp is given using a fully qualified path. Put this in a
helper function and only call it from the scp tests.
OpenBSD-Regress-ID: 7533dc1c4265c1de716abb062957994195b36df4
POSIX does not require diff to have -N, so compare in both directions
with just -r, which should catch missing files in either directory.
OpenBSD-Regress-ID: 0e2ec8594556a6f369ed5a0a90c6806419b845f7