feat(test): use nvim_exec in helpers.source() #16064

helpers.source() was a hack to work around the lack of anonymous
:source. Its "create tempfile" behavior is not a required part of most
tests that use it.

Some tests still need the old "create tempfile" behavior either because
they test SID behavior, or because of missing nvim_exec features: #16071

runtime/doc/options.txt

@@ -3670,8 +3670,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 		0: never
 		1: only if there are at least two windows
 		2: always
-		3: have a global statusline at the bottom instead of one for
-		   each window
+		3: always and ONLY the last window
 	The screen looks nicer with a status line if you have several
 	windows, but it takes another screen line. |status-line|
 

runtime/doc/repeat.txt

@@ -182,8 +182,7 @@ For writing a Vim script, see chapter 41 of the user manual |usr_41.txt|.
 
 					*:so* *:source* *load-vim-script*
 :[range]so[urce] [file]	Runs |Ex| commands or Lua code (".lua" files) from
-			[file], or from the current buffer if no [file] is
-			given.
+			[file], or current buffer if no [file].
 			Triggers the |SourcePre| autocommand.
 							*:source!*
 :[range]so[urce]! {file}

runtime/doc/vim_diff.txt

@@ -180,6 +180,7 @@ Commands:
   |:Man| is available by default, with many improvements such as completion
   |:sign-define| accepts a `numhl` argument, to highlight the line number
   |:match| can be invoked before highlight group is defined
+  |:source| works with Lua and anonymous (no file) scripts
 
 Events:
   |RecordingEnter|