Add npm script "update-test-repos" to recreate the test-repos directory and freshen the corresponding snapshots.

test/snapshots/markdownlint-test-repos-dotnet-docs.mjs.md

@@ -40,7 +40,6 @@ Generated by [AVA](https://avajs.dev).
     test-repos/dotnet-docs/docs/orleans/tutorials-and-samples/overview-helloworld.md: 17: MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]␊
     test-repos/dotnet-docs/docs/standard/native-interop/tutorial-comwrappers.md: 251: MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]␊
     test-repos/dotnet-docs/includes/core-changes/aspnetcore/3.0/authn-apis-json-types.md: 16: MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]␊
-    test-repos/dotnet-docs/includes/core-changes/aspnetcore/3.0/authn-google-plus-authn-changes.md: 19: MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]␊
     test-repos/dotnet-docs/includes/core-changes/aspnetcore/3.0/kestrel-connection-adapters-removed.md: 25: MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]␊
     test-repos/dotnet-docs/includes/migration-guide/retargeting/networking/only-tls-10-11-12-protocols-supported-systemnetservicepointmanager.md: 11: MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]␊
     test-repos/dotnet-docs/includes/migration-guide/retargeting/security/cspparametersparentwindowhandle-now-expects-hwnd-value.md: 23: MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]␊