Add parameterized tests

[josevalim]

Sometimes you want to run the same tests but with different parameters.
In ExUnit, it is possible to do so by passing a :parameterize key to
ExUnit.Case. The value must be a list of maps which will be the
parameters merged into the test context.

For example, Elixir has a module called Registry, which can have type
:unique or :duplicate, and can control its concurrency factor using
the :partitions option. If you have a number of tests that behave the
same across all of those values, you can parameterize those tests with:

use ExUnit.Case,
  async: true,
  parameterize:
    for(kind <- [:unique, :duplicate],
        partitions <- [1, 8],
        do: %{kind: kind, partitions: partitions})

Then, in your tests, you can access the parameters as part of the context:

test "starts a registry", %{kind: kind, partitions: partitions} do
  ...
end

Use parameterized tests with care:

• Although parameterized tests run concurrently when async: true is also given,
  abuse of parameterized tests may make your test suite slower

• If you use parameterized tests and then find yourself adding conditionals
  in your tests to deal with different parameters, then parameterized tests
  may be the wrong solution to your problem. Consider creating separated
  tests and sharing logic between them using regular functions

[josevalim]

When a test fails, this is what it looks like:

[whatyouhide]

Sweet 🍬

[josevalim]

I have pushed a new implementation that allows parameterized tests to run concurrently. Thank you @sorentwo and @wojtekmach for the feedback.

[v0idpwn]

Not sure if truly better:

Suggested change

	{parameterize, opts} = Keyword.pop(opts, :parameterize, nil)
	unless parameterize == nil or (is_list(parameterize) and Enum.all?(parameterize, &is_map/1)) do
	{parameterize, opts} = Keyword.pop(opts, :parameterize)
	if parameterize && not (is_list(parameterize) and Enum.all?(parameterize, &is_map/1)) do

[josevalim]

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