tests: kill some test servers afterwards to avoid locked logfiles

[mback2k]

Testing this against Azure CI to see how this turns out to avoid these messages showing up after test 1272, 1273 and 1561:

2021-10-27T23:01:59.1511713Z Found File lock of 'C:\__w\1\s\tests\log\https_stunnel.log' (234) by tstunnel.exe (8388)
2021-10-27T23:01:59.1513394Z Found File lock of 'C:\__w\1\s\tests\log\https_stunnel.log' (238) by tstunnel.exe (8388)
2021-10-27T23:01:59.1514580Z Found File lock of 'C:\__w\1\s\tests\log\gophers_server.log' (238) by tstunnel.exe (7688)
2021-10-27T23:01:59.1515684Z Found File lock of 'C:\__w\1\s\tests\log\gophers_server.log' (23C) by tstunnel.exe (7688)

[mback2k]

It seems like the tests mentioned above somehow lock up tstunnel.exe logging. Killing of the related test servers has now removed the corresponding locks and allows runtests.pl to delete the log files without errors.