tests: add fprintf() mock with redirection to io_mock

We've seen somewhat obscure test failures where the real fprintf()
function was passed a fake file returned by the fopen() mock.

Although the code that caused the specific failure was cros-specific,
adding an fprintf() mock should help avoid future debugging.

TEST=ninja test
BRANCH=none
BUG=b:217661133

Change-Id: I3f8594ea24d17436a7932732d9d05416b804dc93
Signed-off-by: Nikolai Artemiev <nartemiev@google.com>
---
M tests/io_mock.h
M tests/meson.build
M tests/tests.c
3 files changed, 16 insertions(+), 0 deletions(-)