ci: Remove integration test serialization (#12079)
## Summary
- Removes the `turborepo-integration-serial` test group and its override
from the nextest config.
## Why
These tests (boundaries, query, ls) were serialized as a speculative
workaround for macOS flakiness in #11128, with an explicit note that it
"likely only masks a root cause." Investigation shows there is no shared
mutable state:
- Each test creates its own tempdir
- Each test spawns its own turbo process
- `TURBO_CONFIG_DIR_PATH` is set per-test to the tempdir
- No port binding, no daemon, no sockets
- No global environment variable manipulation
- Insta snapshot names are unique per test
Additionally, the `test(ls)` filter was substring-matching "ls" in test
names, accidentally catching "false", "fails", "details", etc. —
serializing ~20 completely unrelated tests.
## Testing
If CI passes without flakiness, the serialization was unnecessary. If
flakiness reappears on macOS, we should investigate the actual root
cause rather than re-serializing.
