test: fix flaky test-http-client-timeout-option-with-agent

First commit:

There is no guarantee that a timeout won't be delayed considerably due
to unrelated activity on the host. Instead of checking that the timeout
happens within a certain tolerance, simply check that it did not happen
too soon.

Second commit:

test-http-client-timeout-option-with-agent no longer checks that the
timeout happens within a certain tolerance so it can be moved to the
parallel test suite.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines