Tweak various coverage test files for slightly nicer formatting

For coverage tests, splitting code across multiple lines often makes the
resulting coverage report easier to interpret, so we force rustfmt to retain
line breaks by adding dummy line comments with `//`.

tests/coverage/coroutine.coverage

@@ -10,9 +10,9 @@
    LL|       |// to handle this condition, and still report dead block coverage.
    LL|      1|fn get_u32(val: bool) -> Result<u32, String> {
    LL|      1|    if val {
-   LL|      1|        Ok(1)
+   LL|      1|        Ok(1) //
    LL|       |    } else {
-   LL|      0|        Err(String::from("some error"))
+   LL|      0|        Err(String::from("some error")) //
    LL|       |    }
    LL|      1|}
    LL|       |