Change EndToEndTestServerPush::kNumMaxStreams type to unblock QUICHE merge.

EndToEndTestServerPush::kNumMaxStreams has been changed from const to static
constexpr at cl/312150867.  This CL reverts it to the previous type, because of
mysterious linking errors in Chromium:

ld.lld: error: undefined symbol: quic::test::(anonymous namespace)::EndToEndTestServerPush::kNumMaxStreams
>>> referenced by end_to_end_test.cc:3320
>>> (../../net/third_party/quiche/src/quic/core/http/end_to_end_test.cc:3320)
>>>               obj/net/third_party/quiche/quiche_tests/end_to_end_test.o:(quic::test::(anonymous
>>>               namespace)::EndToEndTestServerPush_ServerPushOverLimitWithBlocking_Test::TestBody())
clang: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

The only place linking fails is line 3320, the other occurrences of
kNumMaxStreams are fine.  This must have something to do with EXPECT_EQ
implementation.

Test-only change

PiperOrigin-RevId: 312191615
Change-Id: Icb931a397586953edffb52657805c0d155451da7

quic/core/http/end_to_end_test.cc

diff --git a/quic/core/http/end_to_end_test.cc b/quic/core/http/end_to_end_test.cc
index 75e6f44..b02799d 100644
--- a/quic/core/http/end_to_end_test.cc
+++ b/quic/core/http/end_to_end_test.cc
@@ -3044,7 +3044,7 @@
 // TODO(b/151749109): Test server push for IETF QUIC.
 class EndToEndTestServerPush : public EndToEndTest {
  protected:
-  static constexpr size_t kNumMaxStreams = 10;
+  const size_t kNumMaxStreams = 10;
 
   EndToEndTestServerPush() : EndToEndTest() {
     SetQuicFlag(FLAGS_quic_enable_http3_server_push, true);