Move test helpers from tests/template.bats to lib/

[mbland]

At least some of the test helpers from tests/template.bats in v1.6.0 should be migrated to lib/.

These will certainly be migrated to lib/bats/helpers:

• [x] skip_if_none_present_on_system
• [x] git_for_windows_native_path
• [x] create_forwarding_script

These may contain useful bits worth extracting, though migration isn't yet certain:

• [x] assert_go_core_unpacked
• [x] create_fake_tarball_if_not_using_real_url
• [x] run_with_download_program

[mbland]

This will get closed by #211. As noted above by the issues linked to this thread, skip_if_none_present_on_system is now and create_forwarding_script are now part of lib/bats/helpers (#192, #198), and git_for_windows_native_path has become @go.native_file_path_or_url in lib/portability (#201).

But of the remaining helpers, assert_go_core_unpacked proved unnecessary, and both create_fake_tarball_if_not_using_real_url and run_with_download_program are too specific to tests/template.bats, I've decided.