[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_stdout_is_terminal

@@ -5,6 +5,7 @@ tempfile=$(mktemp)
 die () { echo "$@" ; cleanup; exit 1; }
 cleanup() { rm -f "$tempfile"; }
 
+: nvm.sh
 \. ../../../nvm.sh
 
 if [ -t 1 ] ; then