Add a test case for extern "C" unsafe to the ui tests

Author: tdittr

tests/ui/parser/issues/issue-87217-keyword-order/wrong-unsafe-abi.rs

@@ -0,0 +1,12 @@
+//@ edition:2018
+
+// There is an order to respect for keywords before a function:
+// `<visibility>, const, async, unsafe, extern, "<ABI>"`
+//
+// This test ensures the compiler is helpful about them being misplaced.
+// Visibilities are tested elsewhere.
+
+extern "C" unsafe fn test() {}
+//~^ ERROR
+
+fn main() {}

tests/ui/parser/issues/issue-87217-keyword-order/wrong-unsafe-abi.stderr

@@ -0,0 +1,8 @@
+error: expected `{`, found keyword `unsafe`
+  --> $DIR/wrong-unsafe-abi.rs:9:12
+   |
+LL | extern "C" unsafe fn test() {}
+   |            ^^^^^^ expected `{`
+
+error: aborting due to 1 previous error
+