Problem: tests: Test_windows_external_cmd_in_cwd() is only run in huge
builds (after 9.1.1947).
Solution: Move it to test_system.vim so that it is run in normal builds.
closes: vim/vim#188532c164f02c6
Problem: Too many delete() calls in tests.
Solution: Use deferred delete where possible.
56564964e6
This includes all changes expect changes in test_startup.vim.
Co-authored-by: Bram Moolenaar <Bram@vim.org>