* test: add appActions, stabilize overlayPlot test
- Adds `waitForPlotsToRender`, a function that waits for all active `.plot` elements on the page to load and draw their series data
- Adds `getCanvasPixels`, a function that takes a canvas selector and retrieves an array of canvas pixel data.
- Modifies `getCanvasPixels` to use `page.evaluateHandle()` so that the canvas handle lifetime exists throughout the test (this was causing flakiness before)
* test: refactor and stabilize `plotRendering` tests
* test: remove redundant test suite
* test: stabilize plot legend color swatch test
* docs: mention `waitForPlotsToLoad()` in e2e docs
* refactor: have getCanvasPixels return actual rgba values
* docs: fix typo
* test: use appAction and fix reload wait condition
* docs: add additional context for `waitForPlotsToRender()`
* refactor: one-liner
* docs: tidy up docs
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