Moves the tests for gr-annotation functions into their own test file and
fixes a subtle bug regarding `splitTextNode`'s Unicode branch.
In the DOM implementation of `node.splitText`, `node` is kept in the DOM
and its `textContent` is modified, whereas the Unicode path of
`splitTextNode` would replace it with an entirely new Text node. This
led to the function behaving differently when the Node contained or
did not contain astral code-points.
With this change, `splitTextNode` more-closely behaves like `splitText`
and this behavior is captured in a new unit-test.
Change-Id: I70460694040ba9a3c49937aaafc9db261ca3be3d
3d2e13c21b