[CFE] Mark two incremental tests slow
[CFE] Mark two incremental tests slow

The tests incremental_dart2js_test and incremental_compiler_leak_test
- taking ~1 minute each locally - often times out at 4 minutes (after
almost being done) on the bots. This CL gives them a little more time
hopefully allowing them to finish.

Fixes https://github.com/dart-lang/sdk/issues/43000

Change-Id: Ic6309ebc1a0c77300f1b4f5262f8d48f4de3e0be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157982
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>
Commit-Queue: Jens Johansen <05223a3c6bdf76af6f84755eae231abeebcf4530@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk