[wasm] WBT tests using BrowserRunner seem to hang on a subsequent dotnet new #97054

WBT tests failing on #95980 . The pattern seems to be a test that uses BrowserRunner, then a subsequent test that uses ToolCommand (usually for dotnet new) gets stuck on waiting for the process to exit.

Notes from some investigation:

• BrowserRunner uses ToolCommand to start a process for dotnet run, but never explicitly tries to close that.
• this might be due to async completions not being able to post due to threadpools blocked?
• There are a couple of Wait and task.Result calls which also block
• Making the whole chain more async friendly, and killing the process in Dispose seems to help, but there is something still left

Note that these are being hit only on #95980, and not on main.