Fix govuk-equilateral-height function usage in shape-arrow helper

[kr8n3r]

In #762 we changed the name of the govuk-equilateral-height function to _govuk-equilateral-height to signify that is is a private function.

Unfortunately we forgot to update the name where it was used to create the open/closed indicator for the details component.

This PR fixes this.

Before:

After:

[36degrees]

Argh, good catch - thanks for that. I think I understand why, but it's pretty annoying that Sass silently outputs the function call verbatim. I wonder if there's a way we could try and catch that if it happens.

[kr8n3r]

i guess only if we wrap it in a custom mixin that check if function exist using sass's built-in function-exists

alternatively, if we use get-function(myfunc) when calling a function, that will throw an error in compilation

[36degrees]

Added a test that greps the CSS for anything matching /govuk-[\w-]+\(.*?\)/g, which should catch most function calls that slip through the net… unless they misspell govuk…

[kr8n3r]

fixed the extra semicolon in the test

[alex-ju]

👍 Tested with the details component and mobile navigation menu button.

[36degrees]

Added a big ole' comment around what the test is looking for