unittest: set -Wno-format-zero-length

This flag is set on other compiler as well for normal builds, and
suppresses a warning for empty print, which is apparently quite common.

The unit test build uses its own flags so set it there as well to get
the same behavior as with the rest of the code base.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

cmake/modules/unittest.cmake

@@ -112,6 +112,7 @@ target_compile_options(test_interface INTERFACE
   ${EXTRA_CFLAGS_AS_LIST}
   $<$<COMPILE_LANGUAGE:CXX>:${EXTRA_CXXFLAGS_AS_LIST}>
   $<$<COMPILE_LANGUAGE:ASM>:${EXTRA_AFLAGS_AS_LIST}>
+  -Wno-format-zero-length
   )
 
 target_link_options(testbinary PRIVATE