Tests and suites do not timeout when a zero timeout is changed to nonzero

[fasttime]

Prerequisites

• Checked that your issue hasn't already been filed by cross-referencing issues with the faq label
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Description

When the timeout of a test or suite is set to zero and then changed again to a positive value, the test or suite does not timeout as expected.

Steps to Reproduce

it.only
(
    'test',
    function (done)
    {
        this.timeout(0);
        this.timeout(10);
        setTimeout(done, 100);
    }
);

Expected behavior: [What you expect to happen]

The test above should timeout and fail.

Actual behavior: [What actually happens]

The test passes.

Reproduces how often: every time

Versions

• The output of mocha --version and node node_modules/.bin/mocha --version: 7.1.2
• The output of node --version: 14.0.0

Additional Information

_enableTimeouts is set to false on the Runnable instance when the timeout changes to 0 but never set back to true again.

[craigtaub]

This seems legit. Ive given this a go here #4260