Fix DiG "timed out" message check in "legacy" system test

Fix DiG "timed out" message check in "legacy" system test

The test expects a "connection timed out" message from DiG when it
experiences a timeout, while the current version of DiG prints just
a "timed out" message, like below:

    ;; communications error to 10.53.0.1#11314: timed out
    ;; communications error to 10.53.0.1#11314: timed out
    ;; communications error to 10.53.0.1#11314: timed out

    ; <<>> DiG 9.19.9-dev <<>> -p 11314 +tries +time +tcp +tries +time @10.53.0.1 dropedns. TXT
    ; (1 server found)
    ;; global options: +cmd
    ;; no servers could be reached

Change the expected string to match the current DiG output.

Use the '-F' switch for "grep" for matching a fixed string.

(cherry picked from commit 35988748f31fbbd23048ce63dced74bd4fd4e0f2)