…when comment argument isn't a string literal (#1652)

- **Explanation**:
This fixes a build error which can arise in the code emitted by
`#expect` when the `comment` argument is not a string literal and the
module the `#expect` appears in has a custom type visible named
`Comment`, conflicting with the testing library's built-in type by that
name.
- **Scope**: Affects a narrow use case of expectations: those which
include custom comment arguments which aren't string literals
- **Issues**: #1641
- **Original PRs**: #1645
- **Risk**: Low; affects a corner case in the expansion of `#expect` and
`#require` macros
- **Testing**: Added a new regression test
- **Reviewers**: @grynspan

…d `@Tag`. (#1653)

- **Explanation**: Disallow (warn) the use of the generic clause syntax
on our attribute macros as we never intended for them to work and they
are not handled by the macro expansions.
- **Scope**: Macro expansion
- **Issues**: N/A
- **Original PRs**: #1650
- **Risk**: Low (adds a warning)
- **Testing**: New unit test
- **Reviewers**: @stmontgomery @harlanhaskins

…t output streams. (#1695)

- **Explanation**: Fixes an off-by-one error that can corrupt output
from exit tests.
- **Scope**: Exit tests.
- **Issues**: #1693
- **Original PRs**: #1694
- **Risk**: Low
- **Testing**: New unit test.
- **Reviewers**: @stmontgomery @dimitribouniol