std: adjust str::test_add so that the macro expands to all 3 items (#8012).

Closes #3682.
testsuite: remove 2 very outdated testcases; functionality is tested by other class-*.rs tests.

Fixes #7305, #7307.
testsuite: remove incorrect section of 2 test cases. (Also, &const is disappearing.)

Fixes #7304.
testsuite: add explanation to a Note, and remove duplicated code.

Fixes #7302.
auto merge of #8305 : huonw/rust/triage-fixes, r=cmr

The two deletions are because the test cases are very old (still using `class` and modes!), and, as far as I can tell (since they are so old), the areas they test are well tested by other rpass tests.