Fixed test session issues

[cvpoienaru]

Description

This PR fixes a couple of issues related to test sessions:

• Spawning a test session when either the runsettings or the test sources are incompatible with test session criteria (e.g.: the use of Fakes or any other data collector);
• Aborting test discovery/run operations because of an unhandled exception when returning a non-existent proxy, caused by an earlier incompatibility between the session criteria when the session was spawned and the current operation criteria. This scenario was taken into account before and the solution was to spawn an on-demand testhost that meets the discovery/run criteria, but it wasn't properly tested;
• During the abort scenario described above, a testhost Close call is badly handled and has the effect of leaving on-demand testhosts hanging indefinitely. This issue has been addressed by the current PR, too;

[Evangelink]

A few comments and nits. I am not asking for tests as you told me that you looked into it and it's not easy to add them because of current architecture.

[Evangelink]

LGTM, might be worth asking for a second opinion.

[cvpoienaru]

To be honest, I'd rather just merge since the PR has been up for review since Monday and nobody else weighed in.