Let tests take their DDS domain from RUSTDDS_TEST_DOMAIN_ID

[johannesschrimpf]

The tests create their DomainParticipants on fixed domain ids, most on
domain 0. On a machine that shares a network with other DDS systems,
cargo test joins their domain 0; a robot on my network discovered the
test participants, could not reach the addresses they advertised, and
logged a failed send per sample.

RUSTDDS_TEST_DOMAIN_ID now sets a base added to the domain id of each
test participant. Unset or empty means 0, so CI is unchanged. Tests keep
their current id as an offset (0, or 11, 51, 52 for the ones that stay
apart), so they remain on separate domains after the shift. A value that
is not a u16, or above 147, fails the test with a message naming the
variable; 147 keeps the highest test domain (147 + 52) below the
lock-order test's scratch domains 200-229.

• src/test/test_domain.rs: the helper and parser, included by the
  integration tests through tests/common/mod.rs.
• 23 participant constructions in unit and integration tests use it.
• The raw-socket unit tests (four of which also create a participant)
  derive their fixed ports from the test domain too: the ports of domains
  5, 12, 14, 15 and 16 shift with the participants and stay off their
  ports, and 7412 (participant 1's discovery port) moves to the shifted
  domain.
• Doc tests are not moved: 57 of the 58 doc tests create a participant on
  domain 0, so plain cargo test still joins domain 0; cargo test --tests leaves them out. Letting the doc examples read the variable
  too would change the rendered examples, so that is left as a possible
  follow-up.
• Not touched: tests/lock_order_discovery_db.rs, which already picks its
  own scratch domains (200-229, never 0) and passes them to its child
  through RUSTDDS_LOCK_ORDER_DOMAIN; and the programs in examples/,
  which are not tests.
• README: a short "Running the tests" section.
• Do not drop pending SEDP samples when a new participant is discovered #447 adds a test on domain 47; whichever lands second switches it to
  test_domain_id(47).

Validation (macOS): cargo test --tests -- --test-threads=1 without the
variable and with 12, 14, 16, 42 and 147 (694 unit, 2 + 2 integration
tests), with 42 also with --features security and --features mio_08,
all green; 148 and abc fail with the message; cargo test --doc (58)
green; lsof during the run with 14 showed no port of domain 0 open;
cargo clippy --all-targets and cargo +nightly fmt --check clean.

Developed with assistance from Claude Fable 5.1 and OpenAI Codex (gpt-6-astra).

[jhelovuo]

Thank you!