We should try to avoid using Phobos in the DMD testsuite. Maybe we can add a very reduced and minimalistic version of `text`?

Should be preview - not a transition - as this allows earlier merging.

NOTE TO REVIEWERS: All changes in this file are just "dog fooding" interpolated strings.

Please make this a Ddoc comment. I know the other ones are not, but we're trying to improve. I would also flesh out the comment a bit, something like: `true if an interpolated string, i.e. prefixed with 'i'`. Please also use backticks for code in comments.

Perhaps add tests with other string literal syntaxes as well.