unittests: Pause garbage collector while executing tests

Temporary (?) workaround for currently failing check_alloc_log tests.

test/unit/helpers.lua

@@ -234,7 +234,9 @@ local function gen_itp(it)
       local pid = posix.fork()
       if pid == 0 then
         posix.close(rd)
+        collectgarbage('stop')
         local err, emsg = pcall(func)
+        collectgarbage('restart')
         emsg = tostring(emsg)
         if not err then
           posix.write(wr, ('-\n%05u\n%s'):format(#emsg, emsg))