[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_current

@@ -6,6 +6,7 @@ TEST_DIR="$TEST_PWD/nvm_ls_current_tmp"
 cleanup() { rm -rf "$TEST_DIR"; unset -f return_zero; alias node='node' ; unalias node; }
 die () { echo "$@" ; cleanup ; exit 1; }
 
+: nvm.sh
 \. ../../../nvm.sh
 
 return_zero () { return 0; }