This allows requesting environment variable values set in the C++ test
code, from python mock code. Use case is cross-checking test
results against values expected by the host side part of the test.
* tests/device/Makefile: use bourne shell compatible "." instead ot "source" (ubuntu's dash does not understand it)
* tests/device: make BEGINTIMEOUT a new error return value instead of a test-process-breaking exception
