REGTEST: fix host part in balance-uri-path-only.vtc

We used to set it to ${h1_px_addr} but it randomly fails on certain
hosts (FreeBSD and OSX) where the address is surprisingly set to "::1"
while the Host field contains 127.0.0.1 (hence two different address
families). While this is likely a minor issue in vtest, we don't need
to depend on this and can easily hard-code 127.0.0.1 which is already
used in other tests.
diff --git a/reg-tests/balance/balance-uri-path-only.vtc b/reg-tests/balance/balance-uri-path-only.vtc
index 735c036..1a66114 100644
--- a/reg-tests/balance/balance-uri-path-only.vtc
+++ b/reg-tests/balance/balance-uri-path-only.vtc
@@ -40,7 +40,7 @@
 } -start
 
 client c1 -connect ${h1_px_sock} {
-    txreq -url "http://${h1_px_addr}/url1"
+    txreq -url "http://127.0.0.1/url1"
     rxresp
     expect resp.status == 200
     expect resp.http.Server ~ s2
@@ -54,7 +54,7 @@
 } -run
 
 client c3 -connect ${h1_px_sock} {
-    txreq -url "http://${h1_px_addr}/url2"
+    txreq -url "http://127.0.0.1/url2"
     rxresp
     expect resp.status == 200
     expect resp.http.Server ~ s3