Extensions to Clojure(Script)'s unit testing framework.

### EXTENSIONS TO `clojure.test/is`

There are special assertions for testing Clojure(Script) exceptions that have
been created via "ex-info".  The "(is (ex-thrown? ...))" form tests if an
exception of type ExceptionInfo is thrown, abstracting away the specific type
differences between Clojure and ClojureScript:

(is (ex-thrown? (throw (ex-info "test" {}))))

"(is (ex-thrown-with-data? f ...))" does the same thing, and also tests the
data in the exception (as returned by "ex-data") by applying it to the
function "f":

(ex-thrown-with-data?
  #(clojure.core.match/match % {:text "text"} true :else false)
  (throw (ex-info nil {:text "text"})))

### FIXTURES

Fixtures allow one to run code before and after tests, to set up the
context in which tests should be run.  A fixture is just a function
that returns a function that calls the function passed as an argument; i.e.:
```
(defn- fx0
  [t]
  #(do (setup)
       (t)
       (teardown)))
```
Please note that familiar fixtures are subtly different to clojure.test
fixtures in that they must *return* a function; in this way they are more
like Ring middleware, which is a natural expression of functionality that
wraps testing.

This distinction allows fixtures to be parameterizable without
sacrificing composability, e.g. the fixture:
```
(defn- fx1
  [x t]
  #(do (setup x)
       (t)))
```
can be parameterised on a per-test basis using something like:
```
(testing "..."
  (with-fixtures [(fx0) (fx1 ::foo)]
    (is (= ::foo (test-subject ...)))))
```
which allows fixtures to succinctly express variance in test conditions
without having to resort to dynamic vars, atoms or volatiles for that
configuration.