Add documentation of test/benchmark naming pattrn.

tests/HACKME.md

+HACKME
+======
+
+This package is for reusable tests and benchmarks.
+These test and benchmark functions work over the Node and NodeBuilder interfaces,
+so they should work to test compatibility and compare performance of various implementations of Node.
+
+This is easier said than done.
+
+
+Naming conventions
+------------------
+
+### name prefix
+
+All reusable test functions start with the name prefix `TestSpec_`.
+
+All reusable benchmarks start with the name prefix `BenchmarkSpec_`.
+
+The "Test" and "Benchmark" prefixes are as per the requirements of the
+golang standard `testing` package.  They take `*testing.T` and `*testing.B`
+arguments respectively.  They also take at least one interface argument
+which is how you give your Node implementation to the test spec.
+
+The word "Spec" reflects on the fact that these are reusable/standardized tests.
+
+We recommend you copy-paste these method names outright into the package of your Node implementation.
+It's not necessary, but it's nice for consistency.
+(In the future, there may be tooling to help make automated comparisons
+of different Node implementation's relative performance; this would
+necessarily rely on consistent names across packages.)
+
+If your Node implementation package has *more* tests and benchmarks that
+*are not* from this reusable set, that's great -- but don't use the "Spec"
+word as a segment of their name; it'll make processing bulk output easier.
+
+### full pattern
+
+The full pattern is:
+
+`BenchmarkSpec_{Application}_{FixtureCohort}/codec={codec}/n={size}`
+
+- `{Application}` means what feature or big-picture behavior we're testing.
+  Examples include "Marshal", "Unmarshal", "Walk", etc.
+- `{FixtureCohort}` means... well, see the names from the 'corpus' subpackage;
+  it should be literally one of those strings.
+- `n={size}` will be present for variable-scale benchmarks.
+  You'll have to consider the Application and FixtureCohort to understand the
+  context of what part of the data is being varied in size, though.
+- `codec={codec}` is an example of extra info that might exist for some applications.
+  For example, it might include "json" and "cbor" for "Marshal" and "Unmarshal",
+  but will not be seen at all in other applications like "Walk".
+
+The parts after the slash are those which are handled internally.
+For those, you call the `BenchmarkSpec_*` function name (stopping before the first slash),
+and that function will call `b.Run` to make sub-tests for all the variations.
+For example, when you call `BenchmarkSpec_Walk_MapNStrMap3StrInt`, that one call
+will result in a suite of tests for various sizes, each of which will be denoted
+in the output by `BenchmarkSpec_Walk_MapNStrMap3StrInt/n=1`, then `.../n=2`, etc.
+
+### variable scale benchmarks
+
+Some corpuses have fixed sizes.  Some are variable.
+
+With fixed-size corpuses, you'll see an integer in the "FixtureCohort" name.
+For variable-size corpuses, you'll see the letter "N" in place of an integer.
+
+See the docs in the 'corpus' subpackage for more discussion of this.