coverage: Force -Copt-level=2 in the unreachable-code test

This test is mainly for detecting problems triggered by MIR optimizations, but
the run-coverage tests don't enable optimization by default.

(The coverage-map tests do enable optimization by default, but I'm updating the
test anyway so that the two `unreachable.rs` files don't drift out of sync.)

Author: Zalathar

tests/coverage-map/unreachable.rs

@@ -1,6 +1,6 @@
 #![feature(core_intrinsics)]
 #![feature(coverage_attribute)]
-// compile-flags: --edition=2021
+// compile-flags: --edition=2021 -Copt-level=2
 
 // <https://github.com/rust-lang/rust/issues/116171>
 // If we instrument a function for coverage, but all of its counter-increment

tests/run-coverage/unreachable.coverage

@@ -1,6 +1,6 @@
    LL|       |#![feature(core_intrinsics)]
    LL|       |#![feature(coverage_attribute)]
-   LL|       |// compile-flags: --edition=2021
+   LL|       |// compile-flags: --edition=2021 -Copt-level=2
    LL|       |
    LL|       |// <https://github.com/rust-lang/rust/issues/116171>
    LL|       |// If we instrument a function for coverage, but all of its counter-increment

tests/run-coverage/unreachable.rs

@@ -1,6 +1,6 @@
 #![feature(core_intrinsics)]
 #![feature(coverage_attribute)]
-// compile-flags: --edition=2021
+// compile-flags: --edition=2021 -Copt-level=2
 
 // <https://github.com/rust-lang/rust/issues/116171>
 // If we instrument a function for coverage, but all of its counter-increment