webpack: parameterize build sources

[pjonsson]

What this PR does

TerriaJS-part of TerriaJS/TerriaMap#730

Add a new parameter that is a list
of paths that are included in the js/ts
rule for building, and pass in the test
directory through that path.

This allows TerriaMap to avoid building
the test sources, which means it no longer
requires TerriaJS's devDependencies to be
installed.

Test me

In TerriaMap with TerriaJS/TerriaMap#740 applied: remove fetch-mock from package.json followed by running yarn install, and then run yarn gulp release and see that TerriaMap builds.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.