Added some unit tests as requested

As discussed in PR #78267, for example:

* https://github.com/rust-lang/rust/pull/78267#discussion_r515404722
* https://github.com/rust-lang/rust/pull/78267#discussion_r515405958

compiler/rustc_mir/src/transform/coverage/test_macros/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+authors = ["The Rust Project Developers"]
+name = "coverage_test_macros"
+version = "0.0.0"
+edition = "2018"
+
+[lib]
+proc-macro = true
+doctest = false
+
+[dependencies]
+proc-macro2 = "1"

compiler/rustc_mir/src/transform/coverage/test_macros/src/lib.rs

@@ -0,0 +1,8 @@
+use proc_macro::TokenStream;
+
+#[proc_macro]
+pub fn let_bcb(item: TokenStream) -> TokenStream {
+    format!("let bcb{} = graph::BasicCoverageBlock::from_usize({}); let _ = {};", item, item, item)
+        .parse()
+        .unwrap()
+}