merge log panels 2

[an-dr-eas-k]

This PR is a follow up on #293 . Here only the changes concerning log output are contained.

The features are:

• live log during changes of dotnet test stdout
• merged output inside the single ".NET Test Explorer" output panel.
• the extension logs are prefixed by [info],... tag, while the stdout of dotnet test is plain
• option to clear the panel before test execution.

For this PR I squashed all commits in #295