[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/Unit tests/nvm ls-remote

@@ -12,6 +12,7 @@ cleanup() {
   mv "${NVM_DIR}/alias/lts-backup" "${NVM_DIR}/alias/lts"
 }
 
+: nvm.sh
 \. ../../../nvm.sh
 if [ -n "${NVM_COLORS-}" ]; then
   export TEMP_NVM_COLORS=NVM_COLORS