Don't store time function in each cursor animation

[Jacalz]

Description:

This avoids carrying around the baggage of each entry cursor animation having a field for the time value. Lets just define it globally and change it in tests. The compiler should be able to remove the variable in an optimisation pass somewhere.

NOTE: We can likely remove all of these custom time variables when the synctest package lands (Go 1.24 or newer).

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 59.947% (-0.02%) from 59.964%
when pulling 307be7d on Jacalz:no-time-func-per-cursor
into 7fea2eb on fyne-io:develop.