[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/Aliases/'nvm alias' should ignore leading blank lines in the file

@@ -4,6 +4,7 @@ die() { echo "$@" ; exit 1; }
 
 export NVM_DIR="$(cd ../../.. && pwd)"
 
+: nvm.sh
 \. "${NVM_DIR}/nvm.sh"
 \. ../../common.sh