test: use exec_capture() in more places (#22787)

Problem:
Using `meths.exec2("code", { output = true })` is too verbose.

Solution:
Use exec_capture() in more places.

test/functional/options/keymap_spec.lua

@@ -2,7 +2,7 @@ local helpers = require('test.functional.helpers')(after_each)
 local clear, feed, eq = helpers.clear, helpers.feed, helpers.eq
 local expect, command, eval = helpers.expect,  helpers.command,  helpers.eval
 local insert, call = helpers.insert, helpers.call
-local funcs, dedent = helpers.funcs, helpers.dedent
+local exec_capture, dedent = helpers.exec_capture, helpers.dedent
 
 -- First test it's implemented using the :lmap and :lnoremap commands, then
 -- check those mappings behave as expected.
@@ -30,7 +30,7 @@ describe("'keymap' / :lmap", function()
       command('lmapclear <buffer>')
       command('set keymap=dvorak')
       command('set nomore')
-      local bindings = funcs.nvim_exec2('lmap', { output = true }).output
+      local bindings = exec_capture('lmap')
       eq(dedent([[
 
       l  "            @_