[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_get_download_slug

@@ -7,6 +7,7 @@ cleanup() {
 
 die () { cleanup; echo "$@" ; exit 1; }
 
+: nvm.sh
 \. ../../../nvm.sh
 
 [ "$(nvm_get_download_slug 2>/dev/null ; echo $?)" = '1' ] || die 'invalid flavor did not fail with exit code 1'