Added jest eslint plugin and updated the tests to use best practices

assets/.eslintrc

assets/package-lock.json

assets/package.json

assets/src/scripts/ajax.test.js

assets/src/scripts/d3-rendering/axes.test.js

assets/src/scripts/d3-rendering/tick-marks.test.js

...-location/components/daily-value-hydrograph/index.test.js

...ents/daily-value-hydrograph/selectors/legend-data.test.js

...daily-value-hydrograph/selectors/time-series-data.test.js

...mponents/daily-value-hydrograph/time-series-graph.test.js

...ing-location/components/hydrograph/download-links.test.js

...ing-location/components/hydrograph/graph-controls.test.js

...s/monitoring-location/components/hydrograph/index.test.js

.../monitoring-location/components/hydrograph/legend.test.js

...ring-location/components/hydrograph/method-picker.test.js

...tion/components/hydrograph/selectors/drawing-data.test.js

...ation/components/hydrograph/selectors/legend-data.test.js

...-location/components/hydrograph/time-series-graph.test.js

...monitoring-location/components/hydrograph/tooltip.test.js

...s/monitoring-location/components/map/flood-slider.test.js