compiletest: revision-dependent compile-flags are space-sensitive

[RalfJung]

I saw some tests use syntax like [rev_name]compile-flags: ... to compile the same code with different flags for better coverage, so I wanted to use that as well. But some double-checking showed that the flags did not actually seem to have any effect. And indeed, the following test passes, when really it should fail because one revision has bad flags:

// revisions: a b
// [a]compile-flags: -D not_a_lint

// build-pass

fn main() {}

It took me a bit to realize that I made the mistake of writing // [a]compile-flags: instead of //[a]compile-flags:. This is quite surprising because compiletest flag parsing is not very space-sensitive elsewhere (e.g., //compile-flags and // compile-flags both work).