In the commit 22d507d5 ("iproto: don't hang on uncancellable iproto
request") we used TEST_BUILD define which is absent, we only have
a CMake build option with such name. Let's add a define too.

While at it let's also show this flag in the `tarantool.build` table.

Follow-up #8423

@TarantoolBot document
Title: new tarantool.build.test_build flag

It is `true` if `TEST_BUILD` build option is set and `false` otherwise.