Tests: adjusted proxy variables time checks.

Strict ">= 1" check may fail if timekeeping is not precise, and sleep(1)
results in something like 0.999 time in headers.  In particular, this
happens when using clock_gettime(CLOCK_MONOTONIC_COARSE) for ngx_current_msec
on Ubuntu 16.04.  To fix this, sleep(1) replaced with select(1.1).