Enable code coverage for System.Diagnostics.Process

[stephentoub]

Code coverage for System.Diagnostics.Process was previously disabled due to tests hanging when coverage was enabled. The issue appears to stem from the processes launched by the process tests also having profiling enabled, which currently causes problems for the code coverage tool. This commit works around the problem by removing the relevant environment variables from the launched processes so that profiling isn't enabled for them.

Fixes #655

Opened OpenCover/opencover#274 for the tooling issue.

[weshaggard]

cc @pallavit
The change looks good to me but it does kind of suck that we need to hard code those special environment variables.

[stephentoub]

@weshaggard, yeah. I view it as a workaround until the tooling around it can be fixed. The other alternative would be to simply clear all environment variables; I tested that as well and, at least with our current tests, that works fine. I was just concerned that doing so could lead to unforeseen problems.

[stephentoub]

(I'm going to hold off on merging this until @sawilde has had a chance to review the problem, per the discussion in OpenCover/opencover#274.)

[sawilde]

just to let you know the coverage is just shy of 50% - I have a RC that I am just testing locally and I will push with the update nuget package ref later today

[stephentoub]

Terrific. Thanks, @sawilde!

[stephentoub]

With @sawilde's update to OpenCover, I've changed my PR to just change the .csproj to enable code coverage for the project, as the other workaround I'd implemented is now unnecessary.