Add more unit tests

[GretaCB]

Research how to add unit tests for React site: https://gatsby.dev/unit-testing

Also, look into solidifying the jest UI snapshot tests. I noticed they will unexpectedly fail on an updated class name, which I dont think should be the case.

> jest

 FAIL  src/components/__tests__/header.js
  Header
    ✕ renders correctly (48ms)

  ● Header › renders correctly

    expect(received).toMatchSnapshot()

    Snapshot name: `Header renders correctly 1`

    - Snapshot
    + Received

    @@ -1,7 +1,7 @@
      <header
    -   className="css-j8g5jg"
    +   className="css-1r8gbns"
      >
        <div
          className="css-1pgpot"
        >
          <h1>

       9 |       .create(<Header siteTitle="Carol Hansen" />)
      10 |       .toJSON()
    > 11 |     expect(tree).toMatchSnapshot()
         |                  ^
      12 |   })
      13 | })

      at Object.toMatchSnapshot (src/components/__tests__/header.js:11:18)

 › 1 snapshot failed.
Snapshot Summary
 › 1 snapshot failed from 1 test suite. Inspect your code changes or run `npm test -- -u` to update them.

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   1 failed, 1 total

• Verify where these class ids come from, and check if these class Ids are "properties" that should be "asymmetrically matched".

[GretaCB]

I added scaffolding for unit tests in #6, but could definitely use some more.