py/objexcept: Allow compression of exception message text.

The decompression of error-strings is only done if the string is accessed
via printing or via er.args.  Tests are added for this feature to ensure
the decompression works.

tools/tinytest-codegen.py

@@ -89,6 +89,8 @@ exclude_tests = (
     # different filename in output
     "micropython/emg_exc.py",
     "micropython/heapalloc_traceback.py",
+    # don't have emergency exception buffer
+    "micropython/heapalloc_exc_compressed_emg_exc.py",
     # pattern matching in .exp
     "micropython/meminfo.py",
     # needs sys stdfiles