Give test and main tasks better names

Tests now have the same name as the test that they're running (to allow for
easier diagnosing of failure sources), and the main task is now specially named
'<main>' instead of '<unnamed>'.

Closes #10195
Closes #10073
auto merge of #10204 : alexcrichton/rust/better-names, r=brson

Tests now have the same name as the test that they're running (to allow for
easier diagnosing of failure sources), and the main task is now specially named
`<main>` instead of `<unnamed>`.

Closes #10195
Closes #10073