MNT Add new context to behat config

[GuySartorelli]

Needed because of silverstripe/silverstripe-cms#3059 (comment)

Issue

• Move TinyMCE code into its own module silverstripe-htmleditor-tinymce#2

[emteknetnz]

Will need to add a composer dev dependency for silverstripe/silverstripe-htmleditor-tinymce since there are behat tests that rely on it?

Will need a new PR for subsites to do the same?

[GuySartorelli]

We typically accept that silverstripe/installer will be included, don't we?

[emteknetnz]

Yeah, in practice it'll work fine in CI and on our locals, though from memory we still include dev-deps that behat needs, similar to how we do for phpunit which in practice also always has installer too

I might have remember that wrong though. It's certainly less obvious for behat when deps are actually needed