test: replace busted with local harness

Replace the busted-based Lua test runner with a repo-local harness.

The new harness runs spec files directly under `nvim -ll`, ships its own
reporter and lightweight `luassert` shim, and keeps the helper/preload
flow used by the functional and unit test suites.

Keep the file boundary model shallow and busted-like by restoring `_G`,
`package.loaded`, `package.preload`, `arg`, and the process environment
between files, without carrying extra reset APIs or custom assertion
machinery.

Update the build and test entrypoints to use the new runner, add
black-box coverage for the harness itself, and drop the bundled
busted/luacheck dependency path.

AI-assisted: Codex

test/unit/path_spec.lua

@@ -397,14 +397,6 @@ describe('path.c', function()
   setup(function()
     mkdir('unit-test-directory')
     io.open('unit-test-directory/test.file', 'w'):close()
-
-    -- Since the tests are executed, they are called by an executable. We use
-    -- that executable for several asserts.
-    local absolute_executable = arg[0]
-
-    -- Split absolute_executable into a directory and the actual file name for
-    -- later usage.
-    local directory, executable_name = string.match(absolute_executable, '^(.*)/(.*)$') -- luacheck: ignore
   end)
 
   teardown(function()