tests: Redirect to real I/O to support coverage run

Implement a check that redirects mock io functions to the real
implementations. Real I/O functions are needed for the coverage tool to
be able to create and write files.

BUG=None
BRANCH=None
TEST=None

Change-Id: I0817fce6ea0f53a4c127794a0d8246504675f805
Signed-off-by: Evan Benn <evanbenn@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/69539
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>

tests/meson.build

@@ -23,6 +23,7 @@ test_srcs = files(
   'chip.c',
   'chip_wp.c',
   'selfcheck.c',
+  'io_real.c',
 )
 
 if not programmer.get('dummy').get('active')