Use real `async` instead of async.js where applicable.
The `getPluginTests()` function was never truly async anyway because it only
contains calls to synchronous `fs` modules.
This change is only cosmetic. Its aim is do make it easier to understand the
async changes that are going to be merged later on. It was extracted from the
original work from Ray Bellis.
To verify that nothing has changed, you can run the following command on each
file touched by this commit:
npm install uglify-es
diff --unified <(uglify-js --beautify bracketize <BEFORE.js>) <(uglify-js --beautify bracketize <AFTER.js>)
This is a complete script that does the same automatically (works from a
mercurial clone):
```bash
#!/usr/bin/env bash
set -eu
REVISION=<THIS_REVISION>
PARENT_REV=$(hg identify --rev "${REVISION}" --template '{p1rev}')
FILE_LIST=$(hg status --no-status --change ${REVISION})
UGLIFYJS="node_modules/uglify-es/bin/uglifyjs"
for FILE_NAME in ${FILE_LIST[@]}; do
echo "Checking ${FILE_NAME}"
diff --unified \
<("${UGLIFYJS}" --beautify bracketize <(hg cat --rev "${PARENT_REV}" "${FILE_NAME}")) \
<("${UGLIFYJS}" --beautify bracketize <(hg cat --rev "${REVISION}" "${FILE_NAME}"))
done
```
