New test helpers to acquire and release charts
New test helpers to acquire and release charts

Since we changed the way how meta data are stores, now unit tests need to work on real Chart instances. This commit brings some helpers to inject/cleanup HTML canvas and it's wrapper into/from the DOM.

amir-hadzic/Chart.js,touletan/Chart.js,chartjs/Chart.js,thebenedict/Chart.js,nicktiberi/Chart.js,potatopeelings/Chart.js,jtcorbett/Chart.js,simonbrunel/Chart.js,rvaidya/Chart.js,Tutuchan/Chart.js,touletan/Chart.js,nicktiberi/Chart.js,jeffesp/Chart.js,ldwformat/Chart.js,rvaidya/Chart.js,avlcodemonkey/Chart.js,2Ring/Chart.js,chartjs/Chart.js,supernovel/Chart.js,supernovel/Chart.js,chartjs/Chart.js,jtcorbett/Chart.js,salzhrani/Chart.js,nnnick/Chart.js,salzhrani/Chart.js,bokysan/Chart.js,ritesrnjn/charts,amir-hadzic/Chart.js,touletan/Chart.js,potatopeelings/Chart.js,ldwformat/Chart.js,Magnetic/Chart.js,touletan/Chart.js,el-ee/Chart.js,avlcodemonkey/Chart.js,simonbrunel/Chart.js,pombredanne/Chart.js,2Ring/Chart.js,panzarino/Chart.js,ashiguruma/Chart.js,panzarino/Chart.js,ritesrnjn/charts,Magnetic/Chart.js,bokysan/Chart.js,el-ee/Chart.js,chartjs/Chart.js