Previous code could fail silently, only caught with yaml output
mismatches. This uses ErrorOr to be explicit about the error detection;
recovery isn't supported because we should only print valid yaml (in
tests, at least).
Also moves the test helpers from //toolchain/base to
//toolchain/testing.
Switches tree_test to use the matchers so that, on mismatch, gtest
prints a path to the mismatch (the straight value means it was just
printing "not equal").
Addresses
https://github.com/carbon-language/carbon-lang/pull/3217#discussion_r1323586836
