remove the regex check in statusGet UT

It caused failures on CentOS 7 and FreeBSDs 12 and 13.

unknown file: Failure
C++ exception with description "regex_error" thrown in the test body.
[  FAILED  ] CtrlChannelDhcpv4SrvTest.statusGet (3 ms)

https://jenkins.aws.isc.org/job/kea-dev/job/ut-basic/1018/

Closes #2402 (closed).