dm: test: Don't clear global_data in dm_test_uclass_before_ready()

We must not clear global_data even in tests, since the ram_buffer (which
is used by malloc()) will also be lost, and subsequent tests will fail.

Zero only the global_data fields that are required for the test to function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Tested-by: Joe Hershberger <joe.hershberger@ni.com>