convert most tests in tests/ to YAML ztests

[nwt]

• Add package ztest to run formulaic tests defined in YAML files that
  look like this:

    zql: count()

    input: |
      #0:record[i:int64]
      0:[1;]
      0:[2;]

    output: |
      #0:record[count:uint64]
      0:[2;]

• Convert tests in tests/formats and most of those in tests/suite to
  YAML. As a side effect, both "make test-unit" and "make test-system"
  now run all of the converted tests. (test-unit uses
  driver.(*Driver).Run, while test-system uses the dist/zq executable).

---

Future Work

• Move YAML ztests into the packages they're intended to test. (I put them in the same directory as the tests they replaces to make this change easier to digest.)
• Convert most of the tests in proc/ and some of those in filter/ and zio/* to YAML.
• Convert the remaining tests in tests/suite/errors/. (This requires adding something like an "error expected" field to ztest.Ztest.)
• Pick a directory in which to put regression tests.

[mattnibs]

Excellent!

[aswan]

I'm not very familiar with yaml, is !!binary handled by the parser? For bzng data, it would be great if we could write the data out in hex rather than base64. If somebody is looking at a particular test (or diffs to a particular test) it is tedious to be sure, but feasible to read through the byte-by-byte representation of binary data. This isn't really practical with base64.

[nwt]

@aswan: !!binary is handled by the parser. It is, unfortunately, the only means YAML defines for encoding non-UTF-8 byte sequences.

We could always define our own binary encoding above YAML. Do you think that's worth doing?

[aswan]

We could always define our own binary encoding above YAML. Do you think that's worth doing?

Maybe? That could happen in a later follow-up for sure if it seems like it would be worthwhile.