tests: Move current_io to io_mock.c be visible across tests

tests.c is growing and needs to be split, specifically all
libusb wraps need to be extracted into their own file. See later
in the chain a lot more wraps are added for libusb functions. To
be able to split it, current_io needs to be moved one level up,
to be visible across tests. This allows having multiple files with
wraps, and all the wraps can use current_io.

BUG=b:181803212
TEST=builds and ninja test

Change-Id: I5327b5de430afe13a8cc931c8b4b188dcb8c8cf6
Signed-off-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/57915
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

tests/io_mock.h

@@ -95,4 +95,6 @@ struct io_mock {
 
 void io_mock_register(const struct io_mock *io);
 
+const struct io_mock *get_io(void);
+
 #endif