Fix check viewport-unit-variants

[ZhukDmitryOlegovich]

Hello,

I'm new to doing PR in open source, so if I'm doing something wrong, I'll be glad to receive feedback

I decided to add doiuse to my project and there are a couple of things that stop me from using it, so I’m going to make a series of PRs in which I propose corrections or improvements that will help me

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1. Added a test case that doesn’t work for me

   .test {
     /* include `5vb` */
     background-image: url("data:image/svg+xml;base64,PHN2ZyB3aWR0aD0iMTYiIGhlaWdodD0iMTYiIGZpbGw9Im5vbmUiIHhtbG5zPSJodHRwOi8vd3d3LnczLm9yZy8yMDAwL3N2ZyI+PHBhdGggZmlsbC1ydWxlPSJldmVub2RkIiBjbGlwLXJ1bGU9ImV2ZW5vZGQiIGQ9Ik02LjU4NiA4TDQuMjkzIDUuNzA3YS45OTkuOTk5IDAgMTExLjQxNC0xLjQxNEw4IDYuNTg1bDIuMjkzLTIuMjkyYTEgMSAwIDAxMS40MTQgMS40MTNMOS40MTQgOGwyLjI5MyAyLjI5NGExIDEgMCAwMS0xLjQxNCAxLjQxNEw4IDkuNDEzbC0yLjI5MyAyLjI5M2ExIDEgMCAwMS0xLjQxNC0xLjQxNUw2LjU4NiA4eiIgZmlsbD0iIzI3MkIzNyIgZmlsbC1vcGFjaXR5PSIuMiIvPjwvc3ZnPg==")
                                                                                      /* `5vb` here ^^^ */
   }

2. Corrected the viewport-unit-variants check function by fix checkCSSLengthUnits

[clshortfuse]

All looks good. Thank you!