acme: fix autocert TestHTTPHandlerDefaultFallback

The Go 1.25.2 release made net/url stricter about parsing bracketed IPv6
hostnames, and is rejecting some test URLs used in the autocert
TestHTTPHandlerDefaultFallback test with an error about the
colon-separated fields requiring at least one hex digit.

This commit replaces the invalid `xxxx` portion of some test URLS with
valid hex digits, fixing the test regression.

Change-Id: I84c192b1cd6daf53ef4199f7987437fd825f7041
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/710155
Reviewed-by: Roland Shoemaker <[email protected]>
Auto-Submit: Daniel McCarney <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: cpu

acme/autocert/autocert_test.go

@@ -561,9 +561,9 @@ func TestHTTPHandlerDefaultFallback(t *testing.T) {
 		{"GET", "http://example.org/foo?a=b", 302, "https://example.org/foo?a=b"},
 		{"GET", "http://example.org:80/foo?a=b", 302, "https://example.org:443/foo?a=b"},
 		{"GET", "http://example.org:80/foo%20bar", 302, "https://example.org:443/foo%20bar"},
-		{"GET", "http://[2602:d1:xxxx::c60a]:1234", 302, "https://[2602:d1:xxxx::c60a]:443/"},
-		{"GET", "http://[2602:d1:xxxx::c60a]", 302, "https://[2602:d1:xxxx::c60a]/"},
-		{"GET", "http://[2602:d1:xxxx::c60a]/foo?a=b", 302, "https://[2602:d1:xxxx::c60a]/foo?a=b"},
+		{"GET", "http://[2602:d1:abcd::c60a]:1234", 302, "https://[2602:d1:abcd::c60a]:443/"},
+		{"GET", "http://[2602:d1:abcd::c60a]", 302, "https://[2602:d1:abcd::c60a]/"},
+		{"GET", "http://[2602:d1:abcd::c60a]/foo?a=b", 302, "https://[2602:d1:abcd::c60a]/foo?a=b"},
 		{"HEAD", "http://example.org", 302, "https://example.org/"},
 		{"HEAD", "http://example.org/foo", 302, "https://example.org/foo"},
 		{"HEAD", "http://example.org/foo/bar/", 302, "https://example.org/foo/bar/"},