tests: i2c_emul: drop the array designators

Drop the array dasignators as they generate a

warning: array designators are a C99 extension

warning when compiling with clang. Not too sure if this is an actual
problem or if the warning should be disabled instead but since it's just
a test it should be fine to just drop them.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>