Add syntax test for |@| on FreeApplicative

[ceedubs]

This is a test that is fixed by #568. I just thought it might be nice to have a regression test for the specific case that was discovered to not work before that PR.

Big thanks to @stew for the fix and @smungee for reporting the issue.

[codecov-io]

Current coverage is 75.68%

Merging #570 into master will not affect coverage as of 5e131b9

@@            master    #570   diff @@
======================================
  Files          157     157       
  Stmts         2163    2163       
  Branches        68      68       
  Methods          0       0       
======================================
  Hit           1637    1637       
  Partial          0       0       
  Missed         526     526       

Review entire Coverage Diff as of 5e131b9

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[non]

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[fthomas]

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