Use octicon to replace emojis

[akaroml]

This is to request replacing the emojis in the editor view:

VS Code now supports oct icons, and by adopting them, the test status looks more consistent and appealing in the editor window.
https://code.visualstudio.com/api/references/icons-in-labels
https://octicons.github.com/

[jdneo]

So far the octicon icons are only supported in StatusBarItem and QuickPickItem. Code Lens is not supported yet. But I can first replace the icons in the test explorer.

Update

Seems that the Code Lens support will come in next release: microsoft/vscode#19582