fix(create_tflm_tree): remove recent tests from exported tree (tensorflow#2751)

Remove the recently added span_test.cc and static_vector_test.cc from
the files exported by the create_tflm_tree.py project generation process
by adding them to the list of tests in the Makefile. Unit tests are not
meant to be included in exported trees; they may include files that are
not exported.

This change also ensures that these tests are included when `make test`
is run.

BUG=fixes tensorflow#2718

Author: rkuester

tensorflow/lite/micro/tools/make/Makefile

@@ -330,6 +330,8 @@ $(TENSORFLOW_ROOT)tensorflow/lite/micro/micro_resource_variable_test.cc \
 $(TENSORFLOW_ROOT)tensorflow/lite/micro/micro_time_test.cc \
 $(TENSORFLOW_ROOT)tensorflow/lite/micro/micro_utils_test.cc \
 $(TENSORFLOW_ROOT)tensorflow/lite/micro/recording_micro_allocator_test.cc \
+$(TENSORFLOW_ROOT)tensorflow/lite/micro/span_test.cc \
+$(TENSORFLOW_ROOT)tensorflow/lite/micro/static_vector_test.cc \
 $(TENSORFLOW_ROOT)tensorflow/lite/micro/arena_allocator/non_persistent_arena_buffer_allocator_test.cc \
 $(TENSORFLOW_ROOT)tensorflow/lite/micro/arena_allocator/persistent_arena_buffer_allocator_test.cc \
 $(TENSORFLOW_ROOT)tensorflow/lite/micro/arena_allocator/recording_single_arena_buffer_allocator_test.cc \