Register testr as a distutil entry point

pbr contains a testr distutil command, but since it isn't registered
as an entry-point, it won't get used. Also drive-by a fix to
coverage, where we go to the trouble of setting a variable, and then
not using it.

Change-Id: I2227adf3643ffe6bcece0c7d2127d23e20d52f7a