Consider testing loadability of blacklisted packages

[maleadt]

Out of 5k dynamically blacklisted packages (i.e., deemed unreliable by looking a month's worth of PkgEval runs), most (2791) fail during testing. That means it would probably still be valuable to at least try and install + load these packages, but not test them, to at least spot some regressions that would affect them.

Suggested by @IanButterworth