Don't use the root property to determine suite root.

[lddubeau]

The root property is problematic because Mocha sets it true if the
description of a suite is the empty string. However, Mocha runs
perfectly well with suites that have empty names. See:

mochajs/mocha#2755

Instead of relying on the root property, rely on whether the parent
property is set. If not, then we are at the root.

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[lddubeau]

I signed the CLA.

The failing tests are not due to the changes I made but to a misconfiguration of the Travis parameters (which I did not touch).

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