tests/src/ssl_helpers.c depends on functions defined
in library/*.c. If it's complied as an OBJECT with other c files,
cmake complains undefined reference in link stage under programs/.
Therefore, tests/src/test_helpers/ is created to hold c files with
dependency of library/*.c. Besides, tests/src/test_helper/*.c is
separated into another OBJECT, mbedtls_test_helpers, as sources
to build all test suite executables.
In addition, everest header directory is included in case
MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED is enabled.
Signed-off-by: Yanray Wang <yanray.wang@arm.com>
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