[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/Listing versions/Running 'nvm ls' should list versions in the 'versions' directory

@@ -2,6 +2,7 @@
 
 die () { echo "$@" ; exit 1; }
 
+: nvm.sh
 \. ../../../nvm.sh
 \. ../../common.sh