Port `tests/run-make-fulldeps/hotplug_codegen_backend` to ui-fulldeps
This is the last remaining run-make-fulldeps test, which means I actually had to leave behind a dummy README file to prevent compiletest from complaining about a missing directory.
(Removing the run-make-fulldeps suite entirely is non-trivial, so I intend to do so in a separate PR after this one.)
---
I wasn't sure about adding a new kind of aux build just for this one test, so I also tried to just port this test from Makefile to [rmake](https://github.com/rust-lang/rust/issues/121876) instead.
But I found that I couldn't get rmake to fully work for a run-make-fulldeps test, which convinced me that getting rid of run-make-fulldeps is worthwhile.
r? `@jieyouxu`
An example directive is ignore-test. In legacy compiletest style, the header
would be written as
// ignore-test
but in ui_test style, the header would be written as
//@ ignore-test
compiletest is changed to accept only //@ directives for UI tests
(currently), and will reject and report an error if it encounters any
comments // <content> that may be parsed as an legacy compiletest-style
test header. To fix this, you should migrate to the ui_test-style header
//@ <content>.
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