Add UI test for UNIQUE_CFG_CONDITION

tests/ui/non_minimal_cfg.fixed

@@ -0,0 +1,14 @@
+//@run-rustfix
+
+#![allow(unused)]
+
+#[cfg(windows)]
+fn hermit() {}
+
+#[cfg(windows)]
+fn wasi() {}
+
+#[cfg(all(unix, not(windows)))]
+fn the_end() {}
+
+fn main() {}

tests/ui/non_minimal_cfg.rs

@@ -0,0 +1,14 @@
+//@run-rustfix
+
+#![allow(unused)]
+
+#[cfg(all(windows))]
+fn hermit() {}
+
+#[cfg(any(windows))]
+fn wasi() {}
+
+#[cfg(all(any(unix), all(not(windows))))]
+fn the_end() {}
+
+fn main() {}

tests/ui/non_minimal_cfg.stderr

@@ -0,0 +1,28 @@
+error: unneeded sub `cfg` when there is only one condition
+  --> $DIR/non_minimal_cfg.rs:5:7
+   |
+LL | #[cfg(all(windows))]
+   |       ^^^^^^^^^^^^ help: try: `windows`
+   |
+   = note: `-D clippy::non-minimal-cfg` implied by `-D warnings`
+
+error: unneeded sub `cfg` when there is only one condition
+  --> $DIR/non_minimal_cfg.rs:8:7
+   |
+LL | #[cfg(any(windows))]
+   |       ^^^^^^^^^^^^ help: try: `windows`
+
+error: unneeded sub `cfg` when there is only one condition
+  --> $DIR/non_minimal_cfg.rs:11:11
+   |
+LL | #[cfg(all(any(unix), all(not(windows))))]
+   |           ^^^^^^^^^ help: try: `unix`
+
+error: unneeded sub `cfg` when there is only one condition
+  --> $DIR/non_minimal_cfg.rs:11:22
+   |
+LL | #[cfg(all(any(unix), all(not(windows))))]
+   |                      ^^^^^^^^^^^^^^^^^ help: try: `not(windows)`
+
+error: aborting due to 4 previous errors
+