Don't use txprio to open streams in tests

As per spec a client can only send a HEADERS frame to cause a stream to
transition from the "idle" state to "open". A PRIORITY frame can be sent to an
"idle" stream, but it will remain in that state.

rfc7540,l,916,940
https://tools.ietf.org/html/rfc7540#section-5.1

To open a stream the command

txreq -nostrend

can be used. The -nostrend option will ensure that the stream won't transition
to a "half-closed" state.

bin/varnishtest/tests/t02003.vtc

@@ -43,10 +43,10 @@ client c1 {
 		expect goaway.err == PROTOCOL_ERROR
 	} -start
 	stream 3 {
-		txprio
+		txreq
 	} -run
 	stream 1 {
-		txprio
+		txreq
 	} -run
 	stream 0 -wait
 } -run
@@ -63,13 +63,13 @@ varnish v1 -expect MEMPOOL.sess1.live == 0
 
 client c1 {
 	stream 1 {
-		txprio
+		txreq -nostrend
 		txwinup -size 0
 		rxrst
 		expect rst.err == PROTOCOL_ERROR
 	} -run
 	stream 3 {
-		txprio
+		txreq -nostrend
 		txwinup -size 0x40000000
 		txwinup -size 0x40000000
 		rxrst
@@ -81,7 +81,7 @@ client c1 {
 		expect goaway.err == FRAME_SIZE_ERROR
 	} -start
 	stream 5 {
-		txprio
+		txreq -nostrend
 		sendhex "000003 08 00 00000005"
 		delay .1
 		sendhex 01
@@ -167,7 +167,7 @@ client c1 {
 		expect goaway.laststream == 1
 	} -start
 	stream 1 {
-		txprio
+		txreq -nostrend
 		sendhex "000008 05 00 00000001 0001020304050607"
 	} -run
 	stream 0 -wait
@@ -200,7 +200,7 @@ client c1 {
 		expect goaway.laststream == 1
 	} -start
 	stream 1 {
-		txprio
+		txreq -nostrend
 		# RST wrong length
 		sendhex "000005 03 00 00000001 0000000800"
 	} -run