[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 system' should work as expected

@@ -7,6 +7,7 @@ cleanup() {
 }
 die() { echo "$@" ; cleanup ; exit 1; }
 
+: nvm.sh
 \. ../../nvm.sh
 
 nvm_has_system_node() { return 0; }