tests: have runtests figure out disabled features

... so that runtests can skip individual test cases that test features
that are explicitly disabled in this build. This new logic is intended
for disabled features that aren't otherwise easily visible through the
curl_version_info() or other API calls.

tests/server/disabled is a newly built executable that will output a
list of disabled features. Outputs nothing for a default build.

Closes #3950

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