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/mod.rs

@@ -5,6 +5,9 @@ mod debug;
 mod graph;
 mod spans;
 
+#[cfg(test)]
+mod tests;
+
 use counters::CoverageCounters;
 use graph::{BasicCoverageBlock, BasicCoverageBlockData, CoverageGraph};
 use spans::{CoverageSpan, CoverageSpans};
@@ -31,7 +34,7 @@ use rustc_span::{CharPos, Pos, SourceFile, Span, Symbol};
 
 /// A simple error message wrapper for `coverage::Error`s.
 #[derive(Debug)]
-pub(crate) struct Error {
+pub(self) struct Error {
     message: String,
 }