This PR implements the functionality described above and hides it behind
the `--experimental-faster-testing` flag of `flutter test`.

### The following are some performance measurements from test runs
conducted on GitHub Actions

run 1 logs:
https://github.com/derekxu16/flutter_test_ci/actions/runs/8008029772/attempts/1
run 2 logs:
https://github.com/derekxu16/flutter_test_ci/actions/runs/8008029772/attempts/2
run 3 logs:
https://github.com/derekxu16/flutter_test_ci/actions/runs/8008029772/attempts/3

**length of `flutter test --reporter=expanded test/animation
test/foundation` step**

run 1: 54s
run 2: 52s
run 3: 56s

average: 54s

**length of `flutter test --experimental-faster-testing
--reporter=expanded test/animation test/foundation` step**

run 1: 27s
run 2: 27s
run 3: 29s

average: 27.67s (~48.77% shorter than 54s)

**length of `flutter test --reporter=expanded test/animation
test/foundation test/gestures test/painting test/physics test/rendering
test/scheduler test/semantics test/services` step**

run 1: 260s
run 2: 270s
run 3: 305s

average: 278.33s


**length of `flutter test --experimental-faster-testing
--reporter=expanded test/animation test/foundation test/gestures
test/painting test/physics test/rendering test/scheduler test/semantics
test/services` step**

from a clean build (right after deleting the build folder):

run 1: 215s
run 2: 227s
run 3: 245s

average: 229s (~17.72% shorter than 278.33s)

Note that in reality, `test/material` was not passed to `flutter test`
in the trials below. All of the test files under `test/material` except
for `test/material/icons_test.dart` were listed out individually

**length of `flutter test --reporter=expanded test/material` step**

run 1: 408s
run 2: 421s
run 3: 451s

average: 426.67s

**length of `flutter test --experimental-faster-testing
--reporter=expanded test/material` step**

run 1: 382s
run 2: 373s
run 3: 400s

average: 385s (~9.77% shorter than 426.67s)

---------
Co-authored-by: Dan Field <dnfield@google.com>