Test a bug in Piecewise._eval_subs
Piecewise._eval_subs doesn't use solve.

Also, the checksol is moved to _eval_cond.
Merge pull request #1944 from placasse/piecewise_eval_subs

Piecewise._eval_subs should not call solve.

There is actually two problems with this call to solve.
For some equations, solve doesn't return all solutions.
The second issue with solve is that it can be slow.