Expression tests should check "serialized"

[jfirebaugh]

#6181 added functionality to the expression tests to verify that the actual "serialized" representation matches an expected value. However, it omitted a serialization mechanism in the JS implementation; the JS test harness ignores the expected value for existing tests, and bootstraps newly added tests with an "expected" serialization that's copied from the input expression.

This introduces a new source of friction in the gl-js/gl-native workflow: it's easy to create a new expression test in gl-js that fails unexpectedly when pulled into gl-native because the serialized representation is not exactly equivalent to the input expression. When this happens, you have to either ignore that test in gl-native (even if it would otherwise be passing), or pause your current work and update the expected serialization in a gl-js PR.

/cc @ChrisLoer

[anandthakker]

Yeah -- it wouldn't be too hard to add Expression#serialize() to GL JS, but as of now, we'd be doing it solely for the purpose of these tests. Still, it's not that much code to add, so it's probably worth doing.