Rollup merge of #153736 - cyrgani:incomplete-2, r=fmease

add test that an incomplete feature emits a warning

Related to rust-lang/compiler-team#974, this PR adds a new test to specifically check that a dummy incomplete feature triggers the `incomplete_features` lint. (since this seemed to be the favored approach on Zulip)

Alternative to rust-lang/rust#153706.

r? fmease

Author: JonathanBrouwer