add offline packages

offline/packages/microsoft.testing.platform.msbuild/1.7.3/PACKAGE.md

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+# Microsoft.Testing
+
+Microsoft Testing is a set of platform, framework and protocol intended to make it possible to run any test on any target or device.
+
+Documentation can be found at <https://aka.ms/testingplatform>.
+
+## About
+
+This package provides MSBuild integration for Microsoft Testing Platform.