[Tests] set $_ before sourcing nvm.sh in fast tests

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.

test/fast/Running 'nvm use iojs' uses latest io.js version

@@ -13,6 +13,7 @@ cleanup() {
   rm -rf "$(nvm_version_path "iojs-${VERSION}")"
 }
 
+: nvm.sh
 \. ../../nvm.sh
 
 nvm deactivate || die "unable to deactivate; current: >$(nvm current)<"