Allows setting `$TESTPATHS` external to `test.sh`. (#2201)
Allows setting `$TESTPATHS` external to `test.sh`. (#2201)

Unlike the `$GOTESTFLAGS` var there was no way to pass in a default
value to `test.sh` via `docker-compose -e` to indicate you only want to
run specific unit tests.

This commit puts the default `$TESTPATHS` into `$DEFAULT_TESTPATHS` and
assigns it to `$TESTPATHS` only if there isn't already a `$TESTPATHS`
value provided.

Example usage, running just the SA unit tests, with the "next" config,
using `-race -v`:

```
  docker-compose run -e BOULDER_CONFIG_DIR="test/config-next"
    -e RUN="unit" -e GOTESTFLAGS="-v -race"
    -e TESTPATHS="github.com/letsencrypt/boulder/sa" boulder ./test.sh
```
letsencrypt/boulder,jsha/boulder,lucasallan/boulder,jcjones/boulder,lucasallan/boulder,jfrazelle/boulder,ZCloud-Firstserver/boulder,ibukanov/boulder,jfrazelle/boulder,jsha/boulder,ZCloud-Firstserver/boulder,sophie-h/boulder,jcjones/boulder,letsencrypt/boulder,benileo/boulder,bmw/boulder,jcjones/boulder,benileo/boulder,bmw/boulder,bmw/boulder,benileo/boulder,lucasallan/boulder,ibukanov/boulder,ibukanov/boulder,sophie-h/boulder,ZCloud-Firstserver/boulder,jsha/boulder,jfrazelle/boulder,ZCloud-Firstserver/boulder,benileo/boulder,sophie-h/boulder,jfrazelle/boulder,letsencrypt/boulder,jcjones/boulder