doth: skip 'doth' test if FIPS mode when DH is broken

'doth' depends on a working DH implementation.

bin/tests/system/doth/prereq.sh

@@ -17,4 +17,10 @@ $FEATURETEST --with-libnghttp2 || {
 	echo_i "This test requires libnghttp2 support." >&2
 	exit 255
 }
+
+$FEATURETEST --have-fips-dh ||  {
+        echo_i "FIPS mode Diffie-Hellman not working - skipping doth test"
+        exit 255
+}
+
 exit 0