Watch:test task references outdated test:generate task

## Problem
With the camelCase refactor (https://github.com/Workiva/wGulp/pull/73), a reference to the old `test:generate` task was left in the `watch:test` task definition, causing all `watch:test` runs to fail.

## Solution
Update references of `test:generate` to `preTest`.

## How to Test / +10
- Pull down this branch
- Navigate to `jspm-testing` example
- `npm install && jspm install && gulp watch:test` (this installs the wGulp#master)
- Note that `gulp watch:test` fails
- `npm install --save-dev ../../../wGulp` (installs the local version of wGulp - the watchTestFix branch)
- run `gulp watch:test` again, should succeed

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@maxwellpeterson-wf 
@trentgrover-wf 
@charliekump-wf