Use FindPython3 when cmake version >= 3.15.0

Signed-off-by: okhowang(王沛文) <okhowang@tencent.com>
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 14a7b7e..bd5ed83 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -17,9 +17,8 @@
     set(libs ${libs} ${ZLIB_LIBRARIES})
 endif(ENABLE_ZLIB_SUPPORT)
 
-find_package(PythonInterp)
-if(NOT PYTHONINTERP_FOUND)
-    message(FATAL_ERROR "Cannot build test suites without Python 2 or 3")
+if(NOT MBEDTLS_PYTHON_EXECUTABLE)
+    message(FATAL_ERROR "Cannot build test suites without Python 3")
 endif()
 
 # Enable definition of various functions used throughout the testsuite
@@ -43,7 +42,7 @@
 
     add_custom_command(
         OUTPUT test_suite_${data_name}.c
-        COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/scripts/generate_test_code.py -f ${CMAKE_CURRENT_SOURCE_DIR}/suites/test_suite_${suite_name}.function -d ${CMAKE_CURRENT_SOURCE_DIR}/suites/test_suite_${data_name}.data -t ${CMAKE_CURRENT_SOURCE_DIR}/suites/main_test.function -p ${CMAKE_CURRENT_SOURCE_DIR}/suites/host_test.function -s ${CMAKE_CURRENT_SOURCE_DIR}/suites --helpers-file ${CMAKE_CURRENT_SOURCE_DIR}/suites/helpers.function -o .
+        COMMAND ${MBEDTLS_PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/scripts/generate_test_code.py -f ${CMAKE_CURRENT_SOURCE_DIR}/suites/test_suite_${suite_name}.function -d ${CMAKE_CURRENT_SOURCE_DIR}/suites/test_suite_${data_name}.data -t ${CMAKE_CURRENT_SOURCE_DIR}/suites/main_test.function -p ${CMAKE_CURRENT_SOURCE_DIR}/suites/host_test.function -s ${CMAKE_CURRENT_SOURCE_DIR}/suites --helpers-file ${CMAKE_CURRENT_SOURCE_DIR}/suites/helpers.function -o .
         DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/scripts/generate_test_code.py mbedtls ${CMAKE_CURRENT_SOURCE_DIR}/suites/helpers.function ${CMAKE_CURRENT_SOURCE_DIR}/suites/main_test.function ${CMAKE_CURRENT_SOURCE_DIR}/suites/host_test.function ${CMAKE_CURRENT_SOURCE_DIR}/suites/test_suite_${suite_name}.function ${CMAKE_CURRENT_SOURCE_DIR}/suites/test_suite_${data_name}.data
     )