Keystore2 Tests: Revisit run_as function.

This CL adds run_as_child which starts a child asynchronously and
returns a handle that allows waiting for and communicating with the
child process.

Test: keystore2_test_utils_test
Change-Id: Ibde858f249c94295b5f1c0c0cb29b8a98fb98a3a
1 file changed