Add `try` and `try_err` helper functions to `test/common.sh` that capture stdout/stderr and exit code from a single invocation, eliminating duplicate command executions in tests.
Convert all existing tests that used the `OUTPUT`/`EXIT_CODE` double-invocation pattern to use the new helpers.
Also fixes a pre-existing bug in the `nvm_die_on_prefix` test where ASCII apostrophes were used instead of U+2019 to match nvm.sh output.
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.
