checks: example-guard lets a run of ./examples/ through #1195

(posted by an agent, not by @lneto)

tools/checks/example-guard.sh refuses a lunatik run or spawn whose script begins with examples/ (:14), and the kernel builds the path as LUA_ROOT followed by the script (lunatik_core.c:236, lunatik_conf.h:71). sudo lunatik run ./examples/filter/sni then loads /lib/modules/lua/./examples/filter/sni.lua without tools/watchdog.sh, and the guard exits 0 on it, on master and on #1173's head. Reading the operand with any leading ./ taken off, (run|spawn)( .*)? (\./)*examples/, and proving it on ./examples/x, which must block, and on tests/x, which must pass, closes it.

Found in the review of #1173.