allow reading test environment from python mocks

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/libraries/BSTest/test/test.py

@@ -0,0 +1,11 @@
+from mock_decorators import setup, teardown, setenv, request_env
+
+@setup('environment variables can be set and read from python')
+def setup_envtest(e):
+    setenv(e, 'VAR_FROM_PYTHON', '42')
+
+
+@teardown('environment variables can be set and read from python')
+def teardown_envtest(e):
+    env_value = request_env(e, 'VAR_FROM_TEST')
+    assert(env_value == '24')