test: reduce `exec_lua` calls

`exec_lua` makes code slighly harder to read, so it's beneficial to
remove it in cases where it's possible or convenient.

Not all `exec_lua` calls should be removed even if the test passes as it
changes the semantics of the test even if it happens to pass.

From https://github.com/neovim/neovim/pull/28155#discussion_r1548185779:

"Note for tests like this, which fundamentally are about conversion, you
end up changing what conversion you are testing. Even if the result
happens to be same (as they often are, as we like the rules to be
consistent if possible), you are now testing the RPC conversion rules
instead of the vim script to in-process lua conversion rules."

From https://github.com/neovim/neovim/pull/28155#discussion_r1548190152:

"A test like this specifies that the cursor is valid immediately and not
after a separate cycle of normal (or an other input-processing) mode."

1 files changed, 2 insertions, 1 deletions

diff --git a/test/functional/lua/diagnostic_spec.lua b/test/functional/lua/diagnostic_spec.lua
index 716c1e0f30..f648c0b215 100644
--- a/test/functional/lua/diagnostic_spec.lua
+++ b/test/functional/lua/diagnostic_spec.lua
@@ -8,6 +8,7 @@ local eq = helpers.eq
 local matches = helpers.matches
 local api = helpers.api
 local pcall_err = helpers.pcall_err
+local fn = helpers.fn
 
 describe('vim.diagnostic', function()
   before_each(function()
@@ -109,7 +110,7 @@ describe('vim.diagnostic', function()
       'DiagnosticVirtualTextOk',
       'DiagnosticVirtualTextWarn',
       'DiagnosticWarn',
-    }, exec_lua([[return vim.fn.getcompletion('Diagnostic', 'highlight')]]))
+    }, fn.getcompletion('Diagnostic', 'highlight'))
   end)
 
   it('retrieves diagnostics from all buffers and namespaces', function()