test(agent): let the system pick the UDP listener port

[youndie]

UdpSenderTest bound its listeners on the hardcoded ports 19317–19319. The same commonTest classes run under jvmTest and linuxX64Test, and in one Gradle invocation those two tasks run at the same time (org.gradle.parallel=true), so two processes shared three ports and whichever bound second failed. Whether it failed depended only on whether the two suites happened to overlap.

The listener now binds port 0 and reads the port back; the sender is pointed at that.

Shown red before, green after

On a Linux x64 host, the native test binary filtered to UdpSenderTest was run 80 times in a loop while :agent:jvmTest --rerun --tests '*UdpSenderTest' ran three times beside it — the overlap the two Gradle tasks produce by chance, made certain:

	jvm runs	native loop
main (b2d268d)	3 of 3 red — java.net.BindException: Address already in use	1 of 80 failed — the binary aborted (core dumped)
this branch	3 of 3 green	80 of 80 ok

And the plain form, ./gradlew :agent:jvmTest --rerun :agent:linuxX64Test --rerun :agent:ktlintCheck, three times on this branch: green 3 of 3 (18 native tests, all 6 jvm classes).

The server has the same shape in UdpReceiverTest (fixed 19313). That one is fixed together with #57, which changes how UdpReceiver gets its socket and lets the test read the bound port back.