[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/Running 'nvm use --save' works with a .nvmrc in the parent dir

@@ -1,4 +1,5 @@
 #!/bin/sh
+: nvm.sh
 \. ../../../nvm.sh
 \. ../../common.sh