test: run keventd from the configured libexec path

make distcheck fails in dbus-device:

    sh: /libexec/finit/keventd: not found

distcheck configures with --prefix=$(distdir)/_inst, so the bundled
helpers install below that prefix, not in /libexec/finit, which the test
had hard coded.

Export the configured pkglibexecdir as FINIT_EXECPATH in test.env and
run keventd from there, the same path finit itself is built with.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/test.env.in

@@ -4,3 +4,4 @@ exec_prefix=@exec_prefix@
 export TESTENV_PATH=/bin:/sbin:/usr/bin:/usr/sbin:"@bindir@":"@sbindir@"
 export FINIT_CONF="@FINIT_CONF@"
 export FINIT_RCSD="@FINIT_RCSD@"
+export FINIT_EXECPATH="@pkglibexecdir@"