My system is running the latest tcl, 8.6.0 and so when doing make test with redis I get:
which: no tclsh8.5 in (/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/bin:/usr/i686-pc-linux-gnu/gcc-bin/4.6.3:/opt/vmware/bin)
You need 'tclsh8.5' in order to run the Redis test

I added a tclsh8.5 symbolic link to my tclsh8.6 and make test worked fine then, so it seems to be fully compatible.

I guess make test should check for either 8.5 or 8.6