[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 install' with '--reinstall-packages-from' requires a valid version

@@ -9,6 +9,7 @@ cleanup () {
 
 export NVM_DIR="$(cd ../.. && pwd)"
 
+: nvm.sh
 \. ../../nvm.sh
 \. ../common.sh