[gdb/testsuite] Disallow single argument in multi_line

[gdb/testsuite] Disallow single argument in multi_line

It's a common mistake of mine to do:
...
set l [list "foo" "bar"]
set re [multi_line $l]
...
and to get "foo bar" while I was expecting "foo\r\nbar", which I get after
doing instead:
...
set re [multi_line {*}$l]
...

Detect this type of mistake by erroring out in multi_line when only one
argument is passed.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-06-08  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (multi_line): Require more than one argument.
	* gdb.base/gdbinit-history.exp: Update multi_line call.
	* gdb.base/jit-reader.exp: Remove multi_line call.
	* gdb.fortran/dynamic-ptype-whatis.exp: Same.