compiletest: crashes tests should mention `COMPILETEST_VERBOSE_CRASHES` if no longer crashes

[jieyouxu]

E.g. if a test doesn't have a main fn it can prevent the test from ICEing because ICEing codepath might not yet be reached before compilation is aborted. In this case, the stderr should be printed to help the person figure out that "I forgor to annotate crate type" is the cause.

[matthiaskrgr]

you can set COMPILETEST_VERBOSE_CRASHES env var which should print stdout, stderr and exit code

[jieyouxu]

Oh right, good point, I forgor this existed. We should (I'll do this later unless you want to work on it):

• Mention this env var in the panic message for when the crashes test no longer crash. (Mention COMPILETEST_VERBOSE_CRASHES on crash test failure #130793)
• Mention this env var in rustc-dev-guide. (Document crashes test suite rustc-dev-guide#2075)

EDIT: ended up duplicating some description of `tests/crashes/README.md in rust-lang/rustc-dev-guide#2075 because I would not have thought to look there for docs specifically.