Replace all instances of platform_default_log() with CTEST_LOG_INFO()

[gapisback]

Also, minor fix in exec_worker_thread() in large_inserts_bugs_stress_test.c to avoid divide-by-0 error when elapsed_ns is too small.

-- Note to @rosenhouse : With this change, all unit-tests added newly by my shared-memory prototype work are changed to consistently use CTEST_LOG_INFO(). This brings them in-sync with recently added unit-test verbosity levels interfaces and behaviour.

Here is the confirmation:

agurajada/gabe/shmem-squash-logging-ovhds-poc

sdb-fdb-build:[16] $ grep -l -w platform_log *.c
sdb-fdb-build:[17] $ grep -l -w platform_default_log *.c

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