Refine Firefox testing configuration

[lexaknyazev]

Newer Firefox versions are 64-bit by default and use c:\\Program Files\\Mozilla Firefox on windows.
Testing scripts should be aware of it:

sdk/tools/testing/dart/configuration.dart

Line 280 in aa0ac1d

System.windows: 'C:\\Program Files (x86)\\Mozilla Firefox\\firefox.exe',

Right now, test.py outputs not very helpful warning when it can't run browser from non-existent path:

2017-10-06 16:03:44.088244 Warning: Browser timed out before getting next test. Restarting