Add bats test suite

Black-box tests that run the script with stub commands on a minimal PATH and temp config/state folders.

tests/stubs/checkupdates

@@ -0,0 +1,8 @@
+#!/bin/bash
+# Prints $STUB_DATA/checkupdates; exits $CHECKUPDATES_RC, else 0 with updates and 2 without, like the real one.
+printf '%s %s\n' "${0##*/}" "$*" >> "$STUB_DATA/calls"
+if [[ -s $STUB_DATA/checkupdates ]]; then
+    printf '%s\n' "$(<"$STUB_DATA/checkupdates")"
+    exit "${CHECKUPDATES_RC:-0}"
+fi
+exit "${CHECKUPDATES_RC:-2}"