Fix flakey tests

[dustinsoftware]

This is caused by ConfigureMockEnvironment setting up the static
property Environment. Since tests run in parallel, there is a race
between which tests that set up different fake environments, causing
occasional failures. Disabling parallelization fixes the issue, and
tests run so fast that this should be ok.

[Daniel15]

Thanks! I think HtmlHelperExtensions is the only part of ReactJS.NET that doesn't inject dependencies into its constructor (since it consists entirely of static extension methods) and thus relies on some global environment. I wonder if that could be changed/fixed.