test: add isolated Bash test harness and project skeleton

tests/mocks/bin/sysctl

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+command_name=${0##*/}
+: "${TEST_MOCK_LOG:?TEST_MOCK_LOG must identify the isolated test log}"
+allow_empty=${MOCK_ALLOW_EMPTY_INVOCATION:-}
+printf '%q' "$command_name" >>"$TEST_MOCK_LOG"; for argument in "$@"; do printf ' %q' "$argument" >>"$TEST_MOCK_LOG"; done; printf '\n' >>"$TEST_MOCK_LOG"
+if [[ -n "$allow_empty" && "$allow_empty" != "$command_name" ]]; then
+  printf 'Unexpected mock invocation: %s does not allow empty calls for %s\n' "$command_name" "$allow_empty" >&2
+  exit 99
+fi
+if [[ $# == 0 && "$allow_empty" == "$command_name" ]]; then
+  exit 0
+fi
+printf 'Unexpected mock invocation: %s received arguments\n' "$command_name" >&2
+exit 99