Some more edits to EnumBase and EnumMaskBase (#6054)

Adds a unit test, and some smaller edits:

- Remove the `=` when defining names, in order to change `}` placement
by clang-format on uses.
- context:
https://github.com/carbon-language/carbon-lang/pull/6053#discussion_r2343423178
- I believe with `EnumBase` that keeping the `=` had been a deliberate
choice, so this PR is intended to confirm that removing it is okay.
- Delete `EnumMaskBase::name`
- context:
https://github.com/carbon-language/carbon-lang/pull/6053#discussion_r2344233707
- We can't just do nothing because `EnumBase::name` uses indexing that's
incompatible with `EnumMaskBase`.
- Some small comment cleanups.
- Tests don't need to be in the `Carbon` namespace anymore, macros work
fine in other namespaces, but it's still the right namespace.
- Documentation on `EnumBase::name` seems to be referring to a prior
structure, wherein we had a macro defining the function instead of the
`Names` array.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parse/node_category.cpp

@@ -8,7 +8,8 @@
 
 namespace Carbon::Parse {
 
-CARBON_DEFINE_ENUM_MASK_NAMES(NodeCategory) = {
-    CARBON_NODE_CATEGORY(CARBON_ENUM_MASK_NAME_STRING)};
+CARBON_DEFINE_ENUM_MASK_NAMES(NodeCategory) {
+  CARBON_NODE_CATEGORY(CARBON_ENUM_MASK_NAME_STRING)
+};
 
 }  // namespace Carbon::Parse