Check compiletest behavior: Revision declared multiple times across separate directives

[fmease]

Compiletest and rruxwry both reject repeatedly declared revisions inside the same directive like in:

//@ revisions: a a

However, what does compiletest do if it's split across separate directives?

//@ revisions: a
//@ revisions: a

rruxwry accepts that and ultimately only declares a single revision called a.
If compiletest accepts it, we should emit a warning under Flavor::Vanilla and eventually reject it under Flavor::Rruxwry.