Add macros to check for successful Result

These macros are meant to be used in tests:

  Result<File> maybe_a_file = OpenFile(...);
  EXPECT_OK(maybe_a_file);

On failure, the error is printed.

There's no equivalent EXPECT_NOT_OK() because it is a testing anti-pattern
which causes tests to pass even when the error changed as a result of a
regression. Use EPECT_EQ(result, Error(...)) instead.

Test: cd system/core && atest
Test: m
Change-Id: Ie26f90d3c62620e7b1f10013829ba43ef5364fe1