Fix broken tests in PHP 5.5

[benlk]

The tests that are currently failing (see https://travis-ci.org/INN/Largo/builds/126662077 ) are related to the update functions, and I'm not sure what's causing the array-to-string conversion errors that are making them fail.

[benlk]

From #1191 (comment):

Okay, yup, the failing test in PHP 5.3.29 and WordPress 4.4.2 here is because of how the test is written, and it's failing only on Travis, not in a 5.3.29/4.4.2 Vagrant.

• rewrite test_largo_update_prominence_term_description_single to get a term from the available terms, use its description in the description-to-be-replaced field. We cannot safely assume that 'Test Description 9' will be available.

.

The tests failing in PHP 5.5, which WPEngine uses, are related to the update functions, and do need fixes.

• array to string conversion
• called a thing statically