Tests for coverage of config.c

[huitema]

No description provided.

[huitema]

This PR was motivated by a first run of test coverage measurements using gcov. The focus is on the libraries picoquic-core, picohttp-core, and picoquic-log. The first measurements show generally good coverage, with exceptions. One of those was picoquic\config.c. The goal of this first PR was to assess how much time it would take to fix coverage for a moderately large file. Answer: about a day.

Test coverage, config.c	Lines	Branches
Before this PR	59.7%	51.0%
After this PR	92.8%	76.0%

As shown in the change log, this was done with a combination of writing new tests, adding negative test cases to existing tests, and removing unused code. The exercise did uncover a number of issues in some rarely used faulty input scenarios, proving that there is merit in measuring code coverage.