ztests: Build POSIX arch extra functionality only if possible

The extra ztest functionality for the posix arch
requires the host libC. Disable it if we are
building with an embedded libC.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

subsys/testsuite/ztest/CMakeLists.txt

@@ -24,7 +24,7 @@ zephyr_library_sources_ifdef(CONFIG_ZTEST_MOCKING src/ztest_mock.c)
 zephyr_library_sources_ifdef(CONFIG_ZTRESS src/ztress.c)
 
 
-if(CONFIG_ARCH_POSIX)
+if(CONFIG_ARCH_POSIX AND CONFIG_EXTERNAL_LIBC)
  zephyr_library_sources_ifdef(CONFIG_ZTEST_NEW_API src/ztest_posix.c)
 else()
  zephyr_library_sources_ifdef(CONFIG_ZTEST_NEW_API src/ztest_defaults.c)