When running script tests directly from source via
test/scripts/run_mocha.ts, test/conductor/paths.ts loads the
source-tree placeholder test/build.json where BUILD_ROOT is
"does-not-exist".

Check that BUILD_ROOT exists before creating the default artifacts
directory in test_config.ts, and check that GEN_DIR exists before
returning a local results.html path in ResultsDbReporter.

Bug: none
Change-Id: I11e6b662cbbf8c7cccbba98ac8785b9758052cc1
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/8631915
Auto-Submit: Nikolay Vitkov <nvitkov@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>