fix: warnings of ShellCheck

SC2155: Declare and assign separately to avoid masking return values.
SC2164: Use cd ... || exit in case cd fails.
SC2230: which is non-standard. Use builtin 'command -v' instead.

tests/syntax-tests/update.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-cd "$(dirname "${BASH_SOURCE[0]}")"
+cd "$(dirname "${BASH_SOURCE[0]}")" || exit
 
 python="python3"
 if ! command -v python3 &>/dev/null; then python="python"; fi