[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 foo' where 'foo' is circular aborts

@@ -8,6 +8,7 @@ cleanup() {
   rm -rf "$(nvm_alias_path)/foo"
 }
 
+: nvm.sh
 \. ../../nvm.sh
 
 nvm_make_alias foo foo