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.
Closes #3750 (closed)