When running acceptance tests on a seleniarm image I noticed that Chromium (used in seleniarm)
formats a value in a style attribute in some cases differently than Chrome.
From observations it seems that Chromium doesn't format it but Chrome always adds spaces between property name and value.
This causes that some assertions e.g. stringContains('border-width: 10px;') may pass in Chrome but fail in Chromium.
This commit adds new method assertCssProperty which checks exact value of applied CSS and fixes test EditorTextInputStylesCest.
[MAILPOET-4029]
