[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_install_binary_extract

@@ -40,6 +40,7 @@ test_archi() {
   [ "$(cat "${NVM_DIR}/versions/node/${version}/bin/node")" = "node ${version}" ] || die "Unable to extract ${ext} file"
 }
 
+: nvm.sh
 \. ../../../nvm.sh
 
 set -ex