REGTESTS: http_upgrade: fix incorrect expectation on TCP->H1->H2

Commit e1b9e1bb1 ("REGTESTS: Add script to tests TCP to HTTP upgrades")
included a mistake in the TCP->H1->H2 test, it expected a close while
it ought to expect a 400 bad req, which is what the mux returns in this
case. It happens that this used to work fine with older versions of
vtest which see the close regardless of the 400, but since Vtest commit
8d6c6bd ("Leak-plugging on barriers"), this doesn't work anymore.

Let's fix this by expecting the proper response. This should be backported
where this regtest is present, but only after verifying that it still
works; indeed at the time of writing it's uncertain whether an earlier
version used to immediately close.

reg-tests/connection/tcp_to_http_upgrade.vtc

@@ -144,7 +144,8 @@ client c_err1 -connect ${h1_err1h1_sock} {
 # TCP > H1 > H2 upgrade not allowed
 client c_err2 -connect ${h1_err2h1_sock} {
     send "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"
-    expect_close
+    rxresp
+    expect resp.status == 400
 } -run