test: fix some type warnings (#37483)

test/unit/preprocess.lua

@@ -163,8 +163,7 @@ end
 --- @param hdr string
 --- @return string[]?
 function Gcc:dependencies(hdr)
-  --- @type string
-  local cmd = argss_to_cmd(self.path, { '-M', hdr }) .. ' 2>&1'
+  local cmd = table.concat(argss_to_cmd(self.path, { '-M', hdr }), ' ') .. ' 2>&1'
   local out = assert(io.popen(cmd))
   local deps = out:read('*a')
   out:close()

test/unit/testutil.lua

@@ -201,8 +201,10 @@ local function is_child_cdefs()
   return os.getenv('NVIM_TEST_MAIN_CDEFS') ~= '1'
 end
 
--- use this helper to import C files, you can pass multiple paths at once,
--- this helper will return the C namespace of the nvim library.
+--- use this helper to import C files, you can pass multiple paths at once,
+--- this helper will return the C namespace of the nvim library.
+---
+--- @param ... string
 local function cimport(...)
   local previous_defines --- @type string
   local preprocess_cache --- @type table<string,string>