Make tests work with OpenBSD's unversioned tmux

This tests for OpenBSD 6.x or later.  We could likely extend the
test to much older OpenBSD versions, but not without OS installs
and testing.

src/tests/e2e-test

@@ -60,6 +60,14 @@ test_exitstatus()
 # Tmux check.
 tmux_check()
 {
+    # OpenBSD tmux does not have '-V'.
+    if [ "$(uname -s)" = "OpenBSD" ]; then
+	openbsd_major="$(uname -r)"
+	openbsd_major="${openbsd_major%%.*}"
+	if [ "${openbsd_major}" -ge 6 ]; then
+	    return 0
+	fi
+    fi
     version=$(tmux -V)
     if [ $? != 0 ]; then
 	error "tmux unavailable\n"