Clarify [slow] condition behavior in test documentation
The [slow] condition is true when COG_TEST_FAST=1 is set (in fast mode).
This means '[slow] skip' translates to 'if in fast mode, skip this slow test'.
Updated documentation to:
- Clarify when each condition evaluates to true
- Explain the relationship between condition state and skip behavior
- Add concrete examples of how to use conditions with skip
- Make the fast mode vs full test mode distinction clearer
