`compiletest` compile-flags forwarding between current test crate and auxiliaries/minicore is iffy

I took a closer look -- as is tradition, this is a bit of a mess in compiletest. The gist though is that make_compile_args is needed for the preset compiler flags that ui (or other test modes) should respectively get, except we don't properly or explicitly tell apart:

1. Compiler flags that should be set for all ui tests (unless overriden), which includes auxiliaries or minicore when built as an auxiliary under the test mode
2. Compiler flags that should be set for the current test crate itself, versus auxiliaries, versus minicore

I would say that we probably should not be forwarding compiler flags in the general case (... or provide an opt-out to stop that behavior), because yeah with something like target features, it's not exactly obvious or easy to disable...

Originally posted by @jieyouxu in https://github.com/rust-lang/rust/issues/147812#issuecomment-3419590599