Test that generate_test_code.py is still compatible with Python 2

When Mbed TLS 2.16 was released, the requirement was Python 2, not
Python 3. Since then, upstream Python 2 support has stopped, but it is
still maintained in some long-term-support distributions. For the sake
of users who build the unit tests in such environments, test that
generate_test_code.py remains compatible with Python 2.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/Makefile b/tests/Makefile
index 79f5b8e..29f2fa1 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -62,7 +62,7 @@
 
 .SILENT:
 
-.PHONY: all check test clean
+.PHONY: all c_files check test clean
 
 all: $(BINARIES)
 
@@ -70,6 +70,7 @@
 	$(MAKE) -C ../library
 
 C_FILES := $(addsuffix .c,$(APPS))
+c_files: $(C_FILES)
 
 # Wildcard target for test code generation:
 # A .c file is generated for each .data file in the suites/ directory. Each .c