Reintroduce missing test-cases for escapeShellCommand.

http://d.puremagic.com/issues/show_bug.cgi?id=9310

They seem to have been accidentally forgotten when we introduced the new std.process module. The tests are important since we already has a regression without them ([Issue 9309](http://d.puremagic.com/issues/show_bug.cgi?id=9309)).