Normalize `nvm_version` output when `nvm_ls` returns "system vX" so alias and .nvmrc resolutions treat system correctly.
Add fast tests for system alias behavior in `nvm ls`, `nvm use`, and `nvm which`.
nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.
- fix test 'Running "nvm ls" should display all installed versions.': only expect versions created by this test, but no versions created by other tests