test: avoid false positives in xpath @class tests

[cwillisf]

Proposed Changes

In selenium-helper.js, adjust XPath expressions that inspect @class to check for word boundaries.

Reason for Changes

The @class attribute can have multiple space-separated values, like "foo2 bar baz" for example. The previous code would check for contains(@class, "foo") which, if intended to check for the "foo" class, would return a false positive in that case. The new code tests for contains(concat(" ", @class, " "), " foo ") -- in other words, it checks if " foo2 bar baz " contains " foo ", which would correctly return false in this example.

I ran into this while updating our tests for the new version of Blockly: a false positive @class test was causing an integration test to fail because it attempted to click on the wrong element.

Test Coverage

Covered by existing tests.