Fix failing Magna Charta test in Safari

[MartinJJones]

What

Fix failing Magna Charta test in Safari:

• Updated the cW function to avoid truncating the calculated width, this matches the default behaviour in Magna Charta which does not restrict the number of decimal places
• Use the toBeCloseTo matcher and set the number of decimal points to check to 4 when checking the calculated width

Why

Fixes: #2229

As mentioned in the GitHub issue issue there is a difference in the number of decimal places a browser will go to, using a chart from the component guide as an example:

Chrome

<div
  class="mc-td mc-bar-cell mc-bar-1 mc-bar-outdented"
  style="width: 2.70833%;" <- 5 decimal places
>
  <span style="margin-left: 100%; display: inline-block;">2</span>
</div>

Safari

<div
  class="mc-td mc-bar-cell mc-bar-1 mc-bar-outdented"
  style="width: 2.708333%;" <- 6 decimal places
>
  <span style="margin-left: 100%; display: inline-block;">2</span>
</div>

Further info

The failing test in Safari highlights a couple of other things we may want to consider in the future, for example:

• We could update how widths are calculated in Magna Charta and restrict the number of decimal places so it is consistent across all browsers, unlikely to cause a noticeable difference visually when converted to pixels
• There are lots of other failings tests in Safari, I've not investigated them further but I will create a new issue for this as it would be good to understand what is causing them and if they need to be fixed in Safari
• Lastly, if the failing Safari tests in Jasmine highlight some differences/bugs in different browsers, we could consider running the tests in different browsers to help make them more visible