subcontext: Change a std::string argument into std::string_view

This change prevents that a later CL will trigger the following warning:

parameter 'test_context' is passed by value and only copied once; consider
moving it to avoid unnecessary copies

Change-Id: If5837ee6438efdf194781de041779c1089897789
Signed-off-by: Bart Van Assche <bvanassche@google.com>