tests: do not assume that `/usr/bin/expect` exists
tests: do not assume that `/usr/bin/expect` exists

On my system, there's no shared `/usr/bin/`. Since the code already
tries to check whether `expect` is in the path, let's use that
information instead of hardcoding the path.

(The `shunit2` tests are OK because they go via `/usr/bin/env`, but that
one doesn't support passing multiple arguments, and that's actually
needed for the `expect -f` shebang. Unless we could depend on a
non-portable argument `--split-string` to GNU coreutils 8.30+ as per
https://stackoverflow.com/a/52979955/2245623).

CESNET/libyang,CESNET/libyang