tests: Enable building with add_subdirectory()

When building Mbed Crypto when including it via CMake's
`add_subdirectory()`, the tests are also built by default. This means
all headers the tests need must be public, in order for the build of the
tests to have access to the headers.

Author: Patater

tests/CMakeLists.txt

@@ -52,7 +52,7 @@ function(add_test_suite suite_name)
     target_include_directories(${exe_name}
         PUBLIC ${MBEDTLS_DIR}/include/
         PUBLIC ${MBEDTLS_DIR}/crypto/include/
-        PRIVATE ${MBEDTLS_DIR}/crypto/library/)
+        PUBLIC ${MBEDTLS_DIR}/crypto/library/)
 
     if(${data_name} MATCHES ${SKIP_TEST_SUITES_REGEX})
         message(STATUS "The test suite ${data_name} will not be executed.")