ignore ui-fulldeps/session-derive-errors.rs on beta and stable

The session-derive-errors test ensures the internal SessionDiagnostic
derive macro outputs the right error messages when misused.

The macro relies on the proc_macro2 crate though, which changes its span
behavior depending on whether the channel is nightly or not. This caused
test failures when bumping the channel from nightly to beta/stable.

Since SessionDiagnostic is internal-only we don't care about its
diagnostics quality outside of nightly, as the compiler itself is
developed on nightly. Thus the easiest solution is to ignore that test
on the beta and stable channels.

This also implements `// only-{channel}` and `// ignore-{channel}` in
compiletest to properly support the change.

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