tests: default to more threads for ui-tests
Benchmarks (tested on i5-7200U, 2 cores, 4 threads)
```
master branch:
cargo test // prime caches
cargo --color=always test 70,39s user 21,91s system 180% cpu 51,035 total
cargo --color=always test 70,77s user 22,13s system 180% cpu 51,579 total
cargo --color=always test 70,97s user 22,12s system 180% cpu 51,673 total
cargo --color=always nextest run 78,74s user 22,27s system 220% cpu 45,829 total
cargo --color=always nextest run 78,46s user 21,92s system 224% cpu 44,674 total
cargo --color=always nextest run 78,31s user 22,21s system 228% cpu 43,909 total
Patched (ui_speedup branch):
cargo test // prime cache
cargo --color=always test 97,51s user 32,02s system 288% cpu 44,905 total
cargo --color=always test 99,19s user 31,91s system 276% cpu 47,436 total
cargo --color=always test 98,47s user 31,84s system 284% cpu 45,744 total
cargo --color=always nextest run 102,18s user 30,80s system 350% cpu 37,902 total
cargo --color=always nextest run 99,75s user 29,86s system 350% cpu 36,935 total
cargo --color=always nextest run 100,36s user 29,93s system 351% cpu 37,061 total
```
changelog: use more threads for running clippys ui-tests for ~10% walltime speedup
7f8760a44c