Problem: Repeated default fields and field-by-field assertions obscure
the values that each functional test is meant to check.

Solution: Add eq_partial() to check selected record fields while
preserving list length and order. Use local helpers to fill fixed
expected defaults where tests need full equality. Keep explicit absence
checks and test-specific values at the call sites.

AI-assisted

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Problem: `nvim_set_keymap` leaks the `callback` `LuaRef` when the
LHS is too long.

Solution: Make `set_maparg_lhs_rhs` transfer `rhs_lua` to
`MapArguments` up front so the caller always owns the ref.

In 3a4a66017b74192caaf9af9af172bdc08e0c1608, 4d3a67cd620152d11ab9b5f5bdd973f84cc2d44b
we renamed "buffer" to "buf" in dict parameters.

This commit also renames such keys in dict return-values.

Omitting replace_keycodes when it is false causes some confusion as its
default value is unclear. In nvim_set_keymap() it defaults to false, but
in vim.keymap.set() it defaults to true when it matters.

Problem: When `nvim_set_keymap` tries to overwrite a `<unique>` mapping,
it throws an error even when called in `pcall`.

Solution: src/nvim/mapping.c:buf_do_map no longer calls `semsg`. Its
callers now decide whether to ignore the error, or use
`semsg` (not caught)/`api_set_error` (caught by `pcall`).