[web] improve fileapi tests.

These tests had several issues. The fixes are aimed to reduce flakiness
and remove invalid configurations where tests are not providing any
value. Some of the issues include:

* some tests interferered with each other causing a 18% flakiness rate
  in chrome: they used a common file system that ended up being shared
  in a single browser instance. This is why some tests failed when they
  find a file exists before it was created [1].

* some tests were not properly using async_helper/async_minitest, so
  they may complete before they finish executing

* DDC no longer supports these APIs due to
  https://github.com/dart-lang/sdk/issues/53864. We don't intend to fix
  those issues, so there is no purpose in running these tests in DDC at this
  time.

[1]: https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8766313714239949697/+/u/test_results/ignored_flaky_test_failure_logs

Change-Id: I1dbb28eba2d962000a4d425563dbc1eb6b55dbed
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332266
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>