valgrind: unflake cache purging test
valgrind: unflake cache purging test

Fixes #569.

pagespeed/ngx_pagespeed,digideskio/ngx_pagespeed,0x0mar/ngx_pagespeed,0x0mar/ngx_pagespeed,wanrui/ngx_pagespeed,wanrui/ngx_pagespeed,pagespeed/ngx_pagespeed,digideskio/ngx_pagespeed