Update testsuite docs
Update testsuite docs

After moved to autotools, the method to run the i3 testsuite is changed.
Update the testsuite document to reflect the change.

stapelberg/i3,i3/i3,stapelberg/i3,stapelberg/i3,cornerman/i3,Airblader/i3,cornerman/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3,Airblader/i3,Airblader/i3,stapelberg/i3,Airblader/i3-original,i3/i3,Airblader/i3-original,i3/i3,Airblader/i3-original,cornerman/i3,i3/i3,cornerman/i3