HTTP/3: add Http3StreamDataVIOAdaptor unit tests

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Http3StreamDataVIOAdaptor has no unit test. Its finalize() drain is
currently exercised only indirectly, through the h3_* autests, which need a
full QUIC stack and so do not run in configurations without a QUIC backend.

This adds a catch2 case, Http3StreamDataVIOAdaptor delivers a multi-frame body intact, which builds an adaptor over a sink VIO, pushes a body across several
DATA frames through handle_frame(), calls finalize(), and asserts the sink
received the whole body -- both sink_reader->read_avail() and
sink_vio.nbytes.

This is test-only. No production behaviour changes.

The drain itself is already correct on master: 3076c17172 (#13213) anchors
_reader in the constructor, before any payload is written, so finalize()
drains the buffer from its head rather than from whatever block is current.

The test is a real regression test for that behaviour, not a tautology. Changing
finalize() to allocate its reader at finalize time instead of using the
anchored _reader reproduces the pre-3076c17172 shape, and the test then
fails on both assertions with 50 == 200 -- only the last block's worth of a
200-byte body reaches the sink.

Note on src/proxy/http3/CMakeLists.txt

This adds two entries to the test_http3 target: the new test file, and
Http3StreamDataVIOAdaptor.cc so the adaptor links.

#13629 adds that same Http3StreamDataVIOAdaptor.cc entry for its own test.
The two changes overlap on that one line, so whichever merges second needs a
one-line conflict resolution. Both are otherwise independent and each builds and
passes on its own against master.

Test

test_http3: 137 assertions in 16 test cases, up from 134 in 15 on master.

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[approve ci autest 0]

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[approve ci autest 2]

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[approve ci autest 3]

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[approve ci freebsd clang-analyzer autest]