Fix test assembly discovery

[drewnoakes]

CI is broken, failing with messages:

##[error]Testhost process for source(s) 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\Microsoft.TestPlatform.CoreUtilities.dll' exited with error: A fatal error was encountered. The library 'hostpolicy.dll' required to execute the application was not found in 'C:\Program Files\dotnet\'.
##[error]Failed to run as a self-contained app.
##[error]  - The application was run as a self-contained app because 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.runtimeconfig.json' was not found.
##[error]  - If this should be a framework-dependent app, add the 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.runtimeconfig.json' file and specify the appropriate framework.
##[error]. Please check the diagnostic logs for more information.
##[error]Testhost process for source(s) 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\Microsoft.TestPlatform.PlatformAbstractions.dll' exited with error: A fatal error was encountered. The library 'hostpolicy.dll' required to execute the application was not found in 'C:\Program Files\dotnet\'.
##[error]Failed to run as a self-contained app.
##[error]  - The application was run as a self-contained app because 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.runtimeconfig.json' was not found.
##[error]  - If this should be a framework-dependent app, add the 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.runtimeconfig.json' file and specify the appropriate framework.
##[error]. Please check the diagnostic logs for more information.
##[error]Testhost process for source(s) 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\Microsoft.VisualStudio.TestPlatform.ObjectModel.dll' exited with error: A fatal error was encountered. The library 'hostpolicy.dll' required to execute the application was not found in 'C:\Program Files\dotnet\'.
##[error]Failed to run as a self-contained app.
##[error]  - The application was run as a self-contained app because 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.runtimeconfig.json' was not found.
##[error]  - If this should be a framework-dependent app, add the 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.runtimeconfig.json' file and specify the appropriate framework.
##[error]. Please check the diagnostic logs for more information.
##[error]Testhost process for source(s) 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.dll' exited with error: A fatal error was encountered. The library 'hostpolicy.dll' required to execute the application was not found in 'C:\Program Files\dotnet\'.
##[error]Failed to run as a self-contained app.
##[error]  - The application was run as a self-contained app because 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.runtimeconfig.json' was not found.
##[error]  - If this should be a framework-dependent app, add the 'D:\a\1\s\MetadataExtractor.Tests\bin\Release\net6.0\testhost.runtimeconfig.json' file and specify the appropriate framework.
##[error]. Please check the diagnostic logs for more information.

I think what's happening is that the default test assembly discovery is matching anything with "test" in its name, and is therefore pulling in Microsoft.TestPlatform.CoreUtilities.dll, Microsoft.TestPlatform.PlatformAbstractions.dll, Microsoft.VisualStudio.TestPlatform.ObjectModel.dll and testhost.dll, none of which can be exectuted for the reasons listed.

This PR makes the test assembly explicit, which should hopefully address the above issues.

Also, run tests in parallel.

[drewnoakes]

Yep, that fixed it. No idea what broke it. Something must have changed on Azure Pipelines.