Hard to fix failing builds

## Environment

dotnet: 5.0.301
vscode: 1.58.0
extension: 0.7.5
os: linux

## Issue

When a build fails (before running tests), a little popup shows:
> Build failed. Fix your build and try to run the test(s) again
Source: .NET Core Test Explorer (Extension)

The output window shows:
> Test run for ProjectName.Foo.BarTests
Executing dotnet build in /path/ProjectName/test/Foo.Tests
Error while executing test command: Error: Build command failed

That isn't helpful because I don't know what to fix. So I need to drop to the shell and do a `dotnet build` to find out.

It would be more helpful if the log window showed the build errors.

(Or even better, filter the build errors to show just the names/paths of the buggy files.)

Reproduce the failure by running `dotnet build` in `/path/ProjectName/test/Foo.Tests` for `ProjectName.Foo.BarTests`, then inspect how the extension reports the failed build in its output window. Done means the output includes the build errors or useful buggy-file names instead of only “Build command failed.”

Written by the indexing model from the issue text.