build: do not include -DBoost_TEST_DYN_LINK in seastar_testing_cflags

[tchaikov]

before this change, we include -DBoost_TEST_DYN_LINK in seastar_testing_cflags. but this macro is not documented by Boost document, what we should use is BOOST_TEST_DYN_LINK if we want to link against Boost.Test's shared library. see https://www.boost.org/doc/libs/1_86_0/libs/test/doc/html/boost_test/utf_reference/link_references/link_boost_test_dyn_link.html

the reason why the tests build, is that they pass

• either link against Boost::dynamic_linking via Seastar's CMake building system
• or pass -DBOOST_ALL_DYN_LINK by themselves
• or pass -DSEASTAR_TESTING_MAIN to use Seastar's main()

but either way, seasetar_testing is not supposed to expose this definition or `BOOST_TEST_DYN_LINK in its compile options.

in this change, we just drop this definition from cflags. as it is never checked by Boost.Test. And tests are supposed to define SEASTAR_TESTING_MAIN by themselves.

[tchaikov]

@scylladb/seastar-maint hello maintainers, could you help review this change?