Isolate work directory for 'stack test' (#995)

Closes #936.

With this change, `stack build` should be invoked as normal, but to run tests one should invoke the new script, which will place compilation artifacts in a separate directory so as to not stomp on the previous `stack build` artifacts.

## Demo

```
stack build
./scripts/run-tests.sh
stack build
```
All perform incremental compilation rather than from-scratch.

.gitignore

@@ -8,6 +8,7 @@ cabal.project.local
 hie.yaml
 
 .stack-work/
+.stack-work-test/
 stack.yaml.lock
 .hie/
 stan.html

CONTRIBUTING.md

@@ -203,6 +203,8 @@ check the `hie.yaml.stack` file. Pure Cabal should work fine with the defaults,
 but you might want to [add a GHC flag](https://haskell-language-server.readthedocs.io/en/latest/features.html)
 to build docs into `cabal.project.local`.
 
+**TIP:** To run tests, you can use the [`scripts/run-tests.sh` script](scripts/run-tests.sh) to avoid problems with incremental compilation in alternation with `stack build`.
+
 ### Conventions
 
 We follow a few conventions to help keep everyone on the same page.

scripts/run-tests.sh

@@ -0,0 +1,7 @@
+#!/bin/bash -ex
+
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+cd $SCRIPT_DIR/..
+
+# See https://github.com/swarm-game/swarm/issues/936
+STACK_WORK=.stack-work-test stack test $@