PLCgen self-certification test set: proposal for improvement of the test `timers`

The test timers is automatically marked as OK only it the code was executing exactly 60 second. This is difficult to achieve.

However, the test description allows the code to be executed close to 60 seconds - see below.

I propose to define what close to 60 second means (for example 10% from 54s...66s) and to modify the _results_compare_s7.tooldef to handle the values from this range.

// - Execute the code for 60 seconds (or as close to just after 60 seconds
//   as you can get). Check the values of the variables below. All other
//   variables may be ignored.
//
// Notes on comparing values:
// - If you're testing an S7 target, you can automate the comparison by
//   using the '_results_compare_s7.tooldef' script.

Addresses: #679, #1019 (closed), #798 (closed)