Add CMSIS bluepill blink.

blue-pill/hello/test/README

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+This directory is intended for PIO Unit Testing and project tests.
+
+Unit Testing is a software testing method by which individual units of
+source code, sets of one or more MCU program modules together with associated
+control data, usage procedures, and operating procedures, are tested to
+determine whether they are fit for use. Unit testing finds problems early
+in the development cycle.
+
+More information about PIO Unit Testing:
+- https://docs.platformio.org/page/plus/unit-testing.html