test2405, 2407: mark tests based on lib2405 flaky

Flaky in macOS CI jobs.

2405:
https://github.com/curl/curl/actions/runs/19448567968/job/55648448197 CM gcc-13 aws-lc
https://github.com/curl/curl/actions/runs/19432797208/job/55595742192 AM clang !ssl
https://github.com/curl/curl/actions/runs/19431697816/job/55591941993 AM clang !ssl !debug brotli zstd
https://github.com/curl/curl/actions/runs/19421214342/job/55558775785 CM llvm@18 GnuTLS !ldap krb5 +examples
https://github.com/curl/curl/actions/runs/19413038235/job/55537174590 CM llvm@18 mbedTLS !ldap brotli zstd MultiSSL AppleIDN

2407:
https://github.com/curl/curl/actions/runs/19462732039/job/55691022408 AM clang !ssl
https://github.com/curl/curl/actions/runs/19440283144/job/55621665647 CM gcc-13 OpenSSL gsasl rtmp AppleIDN SecTrust +examples
https://github.com/curl/curl/actions/runs/19440283144/job/55621665682 AM gcc-13 !ssl !debug
https://github.com/curl/curl/actions/runs/19436530386/job/55608724437 CM gcc-13 aws-lc
https://github.com/curl/curl/actions/runs/19436530386/job/55608724641 CM llvm@18 GnuTLS !ldap krb5 +examples
https://github.com/curl/curl/actions/runs/19435651588/job/55605648449 CM llvm@18 OpenSSL gsasl rtmp AppleIDN SecTrust +examples

Ref: https://testclutch.curl.se/static/reports/results-count.html

Bug: https://github.com/curl/curl/pull/19487#issuecomment-3546858203
Bug: https://github.com/curl/curl/pull/19487#issuecomment-3546921877

Follow-up to 2c7e1792a06b81b2bf41c9d348d374a83621340b #19487
Follow-up to 96a5ce5a82eb0f08650649ffafb7bb4e51fc4444 #19481
Follow-up to c78044c07e97cb720049579f4fe3cab33a7ea8d3 #15146 #15155

Closes #19587
diff --git a/tests/data/test2405 b/tests/data/test2405
index 79f5e33..c9eb3ca 100644
--- a/tests/data/test2405
+++ b/tests/data/test2405
@@ -3,6 +3,7 @@
 <keywords>
 multi
 HTTP
+flaky
 </keywords>
 </info>
 
diff --git a/tests/data/test2407 b/tests/data/test2407
index c103cf7..1794daf 100644
--- a/tests/data/test2407
+++ b/tests/data/test2407
@@ -4,6 +4,7 @@
 multi
 HTTP
 HTTP/2
+flaky
 </keywords>
 </info>