Improve robustness of mbedtls_test_fork_run_child

[gilles-peskine-arm]

mbedtls_test_fork_run_child() was added as part of a security fix in the 1.1.0 release. We rushed that part of the test code a little.

The goal of this issue is to do the robustness improvements discussed during the private review:

• Documentation improvements 1 2
• Code cleanup 1
• Improve error reporting for unexpected events.
• Add metatests.