tests.sh: check for parallel_env requirement (closes #1606) (#1607)

* tests.sh: check for parallel_env requirement (closes #1606)

* fix shlint

tests.sh

@@ -1,5 +1,12 @@
 #!/bin/bash
 
+# Check some basic test dependencies
+if ! command -v env_parallel &> /dev/null; then
+    echo 'Error: The `env_parallel` command could not be found. You should run `setup-dev.sh` to install development dependencies.'
+    echo '(Alternatively, run ./tests.sh with `--serial` to skip using parallel test running. However, if `env_parallel` is missing, it is likely that other dependencies like the `zig` compiler are also missing)'
+    exit
+fi
+
 # Run integration tests
 (cd tests/gdb-tests && ./tests.sh $@)