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).

proxy_variables.t

diff --git a/proxy_variables.t b/proxy_variables.t
index ada3f0a..b6fe3a3 100644
--- a/proxy_variables.t
+++ b/proxy_variables.t
@@ -170,12 +170,12 @@
 
 		if ($uri =~ 'bad' && $once) {
 			$once = 0;
-			sleep 1;
+			select undef, undef, undef, 1.1;
 			next;
 		}
 
 		if ($uri =~ 'header') {
-			sleep 1;
+			select undef, undef, undef, 1.1;
 		}
 
 		print $client <<EOF;
@@ -186,7 +186,7 @@
 EOF
 
 		if ($uri =~ 'body') {
-			sleep 1;
+			select undef, undef, undef, 1.1;
 		}
 
 		print $client 'AND-THIS';