Combine UDP broadcast socket bind tests

[Haihan-Jiang]

Updates #1640.

This combines the two IPv4 UDP broadcast send/receive tests that only differed
by the address used to bind the sending socket.

The change:

• extracts the IPv4 external-networking interface setup into a shared base;
• adds a parameterized fixture over (SocketKind, TestAddress);
• instantiates the broadcast send/receive test with V4Broadcast() and
  V4Any().

Validation:

• git diff --check
• bazel build --nobuild --macos_sdk_version=$(xcrun --show-sdk-version) -- //test/syscalls/linux:socket_ipv4_udp_unbound_external_networking_test
• Linux execution through the gVisor build container:
  ./bazel-bin/test/syscalls/linux/socket_ipv4_udp_unbound_external_networking_test
  passed 38/38 tests.

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