feat: async test case switch

[gijswijs]

Fixes #38

Test-jstransformer applies all tests to each method it can find,
regardless of the method being asynchronous or not. This is a problem
for tests that require a method to be asynchronous as mentioned in issue #38 .
These test cases should only be applied to the asynchronous methods. This
commit checks for the text '_async' in the folder name. If it can find it, only the
asynchronous test cases are performed. Asynchronous methods are tested
on synchronous test cases, because that way around it should work,
regardless of a method being asynchronous.

checkFunctionOutput is now asynchronous. This is done, because it is
not a given that template.fn(locals) returns a string. Hence this test
case can fail because template.fn(locals).trim() returns an error. By
wrapping it in a Promise.resolve() both the situations where
template.fn(locals) returns a string or a Promise are covered. This
is also how the jstranformer library works, so the tests now better
mimmick the behavior of the actual library.

[ci-reporter]

The build is failing

✨ Good work on this PR so far! ✨ Unfortunately, the Travis CI build is failing as of 79ada17. Here's the output:

npm test

> test-jstransformer@1.1.0 test /home/travis/build/jstransformers/test-jstransformer
> node test

 • simple
   ✓ transform has an output format (0ms)
   ✓ transform has input formats (0ms)
   ✓ simple.compile() (0ms)
 • multi
   ✓ transform has an output format (0ms)
   ✓ transform has input formats (0ms)
   • first-case
     ✓ multi.compile() (0ms)
   • second-case
     ✓ multi.compile() (0ms)
 • render
   ✓ transform has an output format (0ms)
   ✓ transform has input formats (0ms)
   ✓ render.render() (0ms)
 • renderAsync
   ✓ transform has an output format (0ms)
   ✓ transform has input formats (0ms)
   ✓ renderAsync.renderAsync() (4ms)
 • renderFile
   ✓ transform has an output format (0ms)
   ✓ transform has input formats (0ms)
   ✓ renderFile.renderFile() (0ms)
 • renderFileAsync
   ✓ transform has an output format (0ms)
   ✓ transform has input formats (0ms)
   ✓ renderFileAsync.renderFileAsync() (0ms)
 • failures
failures/reject-render-async.js
   ✓ reject-render-async.js (114ms)

Total duration 140ms
tests passed

> test-jstransformer@1.1.0 posttest /home/travis/build/jstransformers/test-jstransformer
> xo --space=2 index.js


  index.js:78:13
  ✖   78:13  result is never reassigned. Use const instead.                         prefer-const
  ✖   78:98  Missing semicolon.                                                     semi
  ✖   89:13  result is never reassigned. Use const instead.                         prefer-const
  ✖   89:95  Missing semicolon.                                                     semi
  ✖   89:95  Trailing spaces not allowed.                                           no-trailing-spaces
  ✖  109:30  Use .includes(), rather than .indexOf(), when checking for existence.  unicorn/prefer-includes
  ✖  109:48  Strings must use singlequote.                                          quotes
  ✖  124:34  Use .includes(), rather than .indexOf(), when checking for existence.  unicorn/prefer-includes
  ✖  124:52  Strings must use singlequote.                                          quotes
  ✖  139:29  Use .includes(), rather than .indexOf(), when checking for existence.  unicorn/prefer-includes
  ✖  139:47  Strings must use singlequote.                                          quotes
  ✖  154:33  Use .includes(), rather than .indexOf(), when checking for existence.  unicorn/prefer-includes
  ✖  154:51  Strings must use singlequote.                                          quotes

  13 errors

I'm sure you can fix it! If you need help, don't hesitate to ask a maintainer of the project!

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