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>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/61708
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

tests/meson.build

@@ -56,6 +56,7 @@ mocks = [
   '-Wl,--wrap=fsync',
   '-Wl,--wrap=fread',
   '-Wl,--wrap=fgets',
+  '-Wl,--wrap=fprintf',
   '-Wl,--wrap=fclose',
   '-Wl,--wrap=feof',
   '-Wl,--wrap=ferror',