PLCgen testing for v7.0 release

I've done a check and the output of v6.0 is the same as current develop. However, we still need to do testing for:

• #1109 Update PLCgen test set for new ceil, round, floor, sign, and scale. (!1256 (merged))
• Test updated expr_stdlib for TwinCAT.
• Test updated expr_stdlib for S7-300 and S7-400.
• #1100 Self-certification for S7-1200 and S7-1500.

We should probably first do #1109. And then do #1100. We should also test TwinCAT for the changes due to #1109.

Addresses #679