REGTESTS: shorten a bit the delay for the h1/h2 upgrade test

Commit d6c4ed9a96 ("REGTESTS: h1/h2: Update script testing H1/H2
protocol upgrades") introduced a 0.5 second delay which is higher
than those of most other tests (usually 0.05 or 0.2) and triggers
timeouts on my side. Let's just shorten it to 0.2 since its goal
is only to send data separately.

Note: maybe a barrier approach would be possible, though not
      studied.

reg-tests/http-messaging/protocol_upgrade.vtc

@@ -377,7 +377,7 @@ client c10_h1_post -connect ${hap_frt_h1_post_sock} {
 	  -hdr "upgrade: custom_protocol" \
 	  -hdr "content-length: 50"
 
-        delay 0.5
+        delay 0.2
 
 	send_n 5 "0123456789"