test: fix flaky http(s)-set-timeout-server tests
The tests include a callback that might not be invoked but is wrapped in common.mustCall(). Remove the common.mustCall() wrapper and add a comment explaining that it should not be added.
Fixes: https://github.com/nodejs/node/issues/11768
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
test http https