Fix the "statschannel" system test on Windows

Fix the "statschannel" system test on Windows

The SYSTEMTESTTOP variable is set by bin/tests/system/run.sh.  When
system tests are run on Windows, that variable will contain an absolute
Cygwin path.  In the case of the "statschannel" system test, using the
unmodified SYSTEMTESTTOP variable in tests.sh causes the RNDCCMD
variable to contain an invocation of a native Windows application with
an absolute Cygwin path passed as a parameter, which prevents rndc from
working in that system test.  Until we have a cleaner solution, override
SYSTEMTESTTOP with a relative path to work around the issue and thus fix
the "statschannel" system test on Windows.

(cherry picked from commit 4deb2a48d93b5186e958ddd6a8f5ffc80962a5ff)