Improve workspace_reserve test

The test is now conducted on the session workspace in order to have the
client workspace free to generate vtc.log() output which needs
workspace to format the string.

This made me notice that vtc.log, via WS_Reset(), clears a previous
workspace overflow and I wonder if this is how it should be.

bin/varnishtest/tests/r03131.vtc

@@ -12,13 +12,38 @@ varnish v1 -vcl {
 
 	sub vcl_synth {
 		set resp.http.res1 = vtc.workspace_reserve(client, 1024 * 1024);
-		vtc.workspace_alloc(client, -1);
-		set resp.http.res2 = vtc.workspace_reserve(client, 8);
-		set resp.http.res3 = vtc.workspace_reserve(client, 8);
+
+		# XXX overflow gets cleared by WS_Reset called from std.log()
+		# -> do we want this ?
+
+		vtc.workspace_alloc(session, -16);
+		std.log("res(8) = " + vtc.workspace_reserve(session, 8));
+		std.log("res(15) = " + vtc.workspace_reserve(session, 15));
+		std.log("res(16) = " + vtc.workspace_reserve(session, 16));
+		std.log("res(17) = " + vtc.workspace_reserve(session, 17));
+		std.log("res(8) = " + vtc.workspace_reserve(session, 8));
 	}
 } -start
 
+logexpect l1 -v v1 -g raw {
+	expect * * VCL_Log	{^\Qres(8) = 8.000\E$}
+
+	# XXX these should return 16
+	expect 0 = VCL_Log	{^\Qres(15) = 0.000\E$}
+	expect 0 = VCL_Log	{^\Qres(16) = 0.000\E$}
+
+	expect 0 = VCL_Log	{^\Qres(17) = 0.000\E$}
+
+	# workspace is now overflown, but smaller reservation still succeeds
+	expect 0 = VCL_Log	{^\Qres(8) = 8.000\E$}
+
+	expect * = Error	{^workspace_session overflow$}
+} -start
+
 client c1 {
 	txreq
 	rxresp
+	expect resp.status == 500
 } -run
+
+logexpect l1 -wait