Add EXPECT_TEXT runtime test directive

[danobi]

Similar to #2841, would be great to have an EXPECT_TEXT directive that compared output with contents of of a checked in file. This would make testing the human readable text output of helpers like hist() more tractable.

A good candidate is adding more tests for hist() cuz that's mostly been missing. See #2831

[luigirizzo]

See #2845 for a possible implementation of this and EXPECT_JSON features.

I have used EXPECT_FILE instead of EXPECT_TEXT as keyword, because
the latter seems ambiguous (is the rest of the line plaintext to compare, or a filename?)

But I have no strong opinions and this is a trivial change.

I