fix(coverage): merge duplicate reports (#8942)

Merging multiple runs isn't quite right because we 
rely on a 0 count to signal that a block hasn't been called.

Other tools like c8 expect this to be true as-well so we 
need to do our best to merge coverage files rather 
than duplicating them.

cli/tests/run_coverage.ts

@@ -1,3 +1,3 @@
-import { returnsFoo2 } from "./subdir/mod1.ts";
+import { returnsHi } from "./subdir/mod1.ts";
 
-returnsFoo2();
+returnsHi();