protoutil: Fix EncodedBuffer test

Commit ag/10780537 ("Optimize memory usage in incidentd")
broke the libprotoutil_test since it page aligns the chunk
size but didn't update the tests.

Fix the tests by page aligning TEST_CHUNK_SIZE; and add a
test mapping.

Test: atest -c libprotoutil_test
Bug: 295228590
Change-Id: I85d828a10cc67334751c3c3a451a75c4dbe3eb76
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>