Rename [slow] condition to [fast] for better readability
The [slow] condition was confusing because:
- It was true when COG_TEST_FAST=1 (in fast mode)
- '[slow] skip' read as 'slow skip' but meant 'if fast, skip'
Renamed to [fast] for clarity:
- [fast] is true when COG_TEST_FAST=1 (in fast mode)
- '[fast] skip' reads as 'if fast mode, skip this test'
- Direct mapping between env var name and condition name
Changes:
- Renamed condition in suite_test.go: slow -> fast
- Updated all 13 .txtar test files using the condition
- Updated documentation in CONTRIBUTING.md and README.md
The behavior is unchanged, only the naming is more intuitive.
