Follow up to #19079, which made test names fully qualified.
This fixes tests that now-redundant information in their test names. For example here's a fully qualified test name before the changes in this commit:
"priority_queue.test.std.PriorityQueue: shrinkAndFree"
and the same test's name after the changes in this commit:
"priority_queue.test.shrinkAndFree"
The idea here is to avoid code bloat by having only one actual io.Reader
implementation, which is type erased, and then implement a GenericReader
that preserves type information on top of that as thin glue code.
The strategy here is for that glue code to `@errSetCast` the result of
the type-erased reader functions, however, while trying to do that I
ran into #17343.
