Add Card tests

[seanmturley]

Description

Add tests for the Card component.

A guide to testing React Router related functions is available in the official Testing Library documentation.

Tests:

On subreddit pages Card should display:

• [x] "Posted by [u/author]"
• [x] the post's image or thumbnail
• [x] the correct time since posting
• [x] correct pluralisation of units of time ~and comments/upvotes~
• [x] the post's title
• [x] the number of comments (abbreviated correctly for large numbers)
• [x] the number of upvotes (abbreviated correctly for large numbers)

On the homepage and search pages Card should display:

• [x] "Posted in [r/subreddit]"
• (All other attributes are the same as subreddit pages, and thus redundant)

[seanmturley]

Spent a bit of time working on formatNumber improvements, including correct plural vs singular for "comments"/"upvotes ", but I think these words will be replaced with icons anyway, so it's unnecessary.

I'm removing these from the task list above (though correct pluralisation still applies to the units of time for post age).

[seanmturley]

Added tests for post age - possibly a bit over-exhaustive, though I only checked rounding and removal of plural "s" for one unit of time.

[seanmturley]

Added simple test to check the post's image loads in Card.

That's the Card tests done. Closing.