Consider converting `tests/ui/json/json-bom-plus-crlf-multifile-aux.rs`-associated tests into a `run-make` tests

[jieyouxu]

I found this aux file (and its associated main test file) really awkward to change, because the in-source BOM marker and CRLF line endings are load-bearing. Apparently the byte offsets are, too. Probably use a run-make test instead to "craft" a BOM + CRLF auxiliary instead to:

1. Make it possible to adjust the auxiliary without accidentally regressing the test intention.
2. Have some checks that the auxiliary actually has the BOM marker + CRLF line endings.

That test directory uses .gitattributes to help enforce CRLF line endings, which is clever, but also...

rust/tests/ui/json/json-bom-plus-crlf-multifile-aux.rs

Lines 1 to 13 in 15c4cce

	// (This line has BOM so it's ignored by compiletest for directives)
	//
	//@ ignore-test Not a test. Used by other tests
	// ignore-tidy-cr
	// For easier verifying, the byte offsets in this file should match those
	// in the json-bom-plus-crlf.rs - given the actual fn is identical (just with
	// a different, but equally sized name), the easiest way to do this is to
	// ensure the two files are of equal size on disk.
	// Padding............................
	// N.B., this file needs CRLF line endings. The .gitattributes file in
	// this directory should enforce it.

[jieyouxu]

Noticed while working on #139967.

[jieyouxu]

I was scared to change the comments / compiletest directives because the comment said something about byte offsets, so I didn't bother to investigate further (yet).

[jieyouxu]

Er wait,

 // (This line has BOM so it's ignored by compiletest for directives) 

That is... certainly a behavior.