Integration tests should not create symlinks

[wxtim]

(Unless install symlink creation is under test)

The tests in tests/integration/test_install.py rely on the user disabling Symlink creation in thier global config to pass reliably, else they fail on second run becuase the previous run is not cleaned correctly.

Should we disable symlink installs in integration tests unless actively required?

[!NOTE] This bug is not user facing, and with correct setup of the global.cylc shouldn't affect developers most of the time -> Low priority.

[oliver-sanders]

(removing bug label because, as you say, it's not user-facing)

Enough just to patch the remaining (2?) tests that still do this.