gnulib-tool: add support for --automake-subdir-tests <https://lists.gnu.org/r/bug-gnulib/2022-01/msg00111.html> * gnulib-tool (main): Handle --automake-subdir-tests. (func_emit_shellvars_init, func_emit_lib_Makefile_am): Use $sourcebase when handling tests and --automake-subdir-tests is given. (func_append_actionarg): Support --automake-subdir-tests. (func_create_testdir): Add missing argument for func_emit_initmacro_end.