[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_alias

@@ -6,6 +6,7 @@ cleanup () {
   rm -rf ../../../alias/test
 }
 
+: nvm.sh
 \. ../../../nvm.sh
 
 OUTPUT="$(nvm_alias 2>&1)"