[e2e tests] fix connection failure regex (#36922)

Closes #36922

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36922 from markdroth:e2e_test_flake_fix 1bcc04aaa7
PiperOrigin-RevId: 643176629

test/cpp/end2end/client_lb_end2end_test.cc

@@ -588,7 +588,7 @@ class ClientLbEnd2endTest : public ::testing::Test {
                         "(Failed to connect to remote host: )?"
                         "(Timeout occurred: )?"
                         // Syscall
-                        "((connect|recvmsg|getsockopt\\(SO\\_ERROR\\)): )?"
+                        "((connect|recvmsg|getsockopt\\(SO\\_ERROR\\)): ?)?"
                         // strerror() output or other message
                         "(Connection refused"
                         "|Connection reset by peer"

test/cpp/end2end/xds/xds_end2end_test_lib.cc

@@ -844,7 +844,7 @@ std::string XdsEnd2endTest::MakeConnectionFailureRegex(
                       "(Failed to connect to remote host: )?"
                       "(Timeout occurred: )?"
                       // Syscall
-                      "((connect|recvmsg|getsockopt\\(SO\\_ERROR\\)): )?"
+                      "((connect|recvmsg|getsockopt\\(SO\\_ERROR\\)): ?)?"
                       // strerror() output or other message
                       "(Connection refused"
                       "|Connection reset by peer"