coverage: UI test for unstable value -C instrument-coverage=branch

Author: Zalathar

tests/ui/instrument-coverage/except-unused-functions.rs

@@ -0,0 +1,2 @@
+error: `-C instrument-coverage=branch` and `-C instrument-coverage=except-*` require `-Z unstable-options`
+

tests/ui/instrument-coverage/except-unused-generics.rs

@@ -0,0 +1,6 @@
+// revisions: branch except-unused-functions except-unused-generics
+// [branch] compile-flags: -Cinstrument-coverage=branch
+// [except-unused-functions] compile-flags: -Cinstrument-coverage=except-unused-functions
+// [except-unused-generics] compile-flags: -Cinstrument-coverage=except-unused-generics
+
+fn main() {}