Reuse selenium fixture in tests

Mechanism for testing imports

MAINT Fix emscripten version 1.38.10 everywhere and remove python2 from build

Don't include compiled tests

Fix #109: Fix the path of external data files

Automatically retry failed tests

Save Circle-CI cache every time

Add support for sha256 checksum

Update chromedriver

Use pathlib for path manipulations

Update geckodriver to 0.21.0

Fix #93 by avoiding use of TextDecoder

Any use of TextDecoder on a part of the Wasm HEAP seems to cause crashes
when using WebAssembly.instantiate later on (in Chrome).

The fix is to:

- Avoid use of TextDecoder in emscripten-generated code by setting
  -s TEXTDECODER=0
- Do the Python-to-JS string conversion a different way:

  - Handle the native format of a Python Unicode string (which is either UCS1,
    UCS 2 or UCS 4) directly.  This has the added advantage of being
    computationally simpler than encoding/decoding to/from UTF8.