Fix tests breaking with tilt 2.0.10

[aerostitch]

Hi,

Since upgrading to tilt 2.0.10 the test fail with the following error:

  1) Error:
Slim test suite::HTML tags::Attributes::Ruby attributes#test_0002_should render:
ArgumentError: unknown command 'a'
    (__TEMPLATE__):6:in `test'
    (__TEMPLATE__):6:in `__tilt_47336302970300'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:170:in `call'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:170:in `evaluate'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:109:in `render'
    /tmp/slim/test/literate/helper.rb:13:in `render'
    (eval):436:in `block (6 levels) in <class:LiterateTest>'

  2) Error:
Slim test suite::HTML tags::Attributes::Dynamic tags `*`#test_0001_should render:
NameError: undefined local variable or method `a_unless_current' for #<#<Class:0x0000561aafb97510>:0x0000561aafe192c0>
    (__TEMPLATE__):7:in `__tilt_47336302970300'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:170:in `call'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:170:in `evaluate'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:109:in `render'
    /tmp/slim/test/literate/helper.rb:13:in `render'
    (eval):492:in `block (6 levels) in <class:LiterateTest>'

  3) Error:
Slim test suite::Line indicators::Output `=`#test_0004_should render:
ArgumentError: unknown command 'a'
    (__TEMPLATE__):6:in `test'
    (__TEMPLATE__):6:in `__tilt_47336302970300'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:170:in `call'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:170:in `evaluate'
    /tmp/slim/vendor/bundle/ruby/2.5.0/gems/tilt-2.0.10/lib/tilt/template.rb:109:in `render'
    /tmp/slim/test/literate/helper.rb:13:in `render'
    (eval):149:in `block (5 levels) in <class:LiterateTest>'

This issue is explained in rtomayko/tilt#347 and a workaround is provided there too.
This PR implements that proposed workaround.

Thanks,
Joseph

[aerostitch]

The tests still fail for Rails but that's a totally separate issue.
I fixed a missing file that is now required with Sprockets 4 but that's as far as I can go for now.
Feel free to merge and treat the rails problem separately.
Thanks

[stonean]

Thank you @aerostitch!

[aerostitch]

Thanks for the quick merge! :)