Removes thread check as the thread may be dead

Depending on the rack_options passed into Rack::Server the thread may
not stay running and can simply exit immediately. If that happens then
this will always return false. I doesn't seem to be necessary to check
the thread status before checking if the server is running via TCP so
I've removed this check.
Fixes problems running all tests after success

The instance variables set during initialization were getting overridden
by the last execution which causes the console runner to keep running
tests on the same file for each call to execute. This separates the
values in initialize from those passed to execute to avoid this.
Merge branch 'improvements-for-guard'