Alp Mestanogullari authored Apr 10, 2019 and Marge Bot committed Apr 12, 2019

When -a or --test-accept is passed, and if one runs the 'test' target, then
any test failing because of mismatching output and which is not expected to
fail will have its expected output adjusted by the test driver, effectively
considering the new output correct from now on.

When this flag is passed, hadrian's 'test' target becomes sensitive to the
PLATFORM and OS environment variable, just like the Make build system:
- when the PLATFORM env var is set to "YES", when accepting a result, accept it
  for the current platform;
- when the OS env var is set to "YES", when accepting a result, accept it
  for all wordsizes of the current operating system.

This can all be combined with `--only="..."` and `TEST="..." to only accept
the new output of a subset of tests.