Mock EuiIcon in Jest

[thompsongl]

Summary

Uses the new testenv mock to replace EuiIcon in snapshot test output, eliminating the loading/loaded churn sometimes seen with async usage.

Closes #2765

Typical resulting diff is something like:

-   <svg
+   <div
      aria-hidden="true"
-     class="euiIcon euiIcon--medium euiIcon-isLoading euiButtonIcon__icon"
-     focusable="false"
-     height="16"
-     role="img"
-     viewBox="0 0 16 16"
-     width="16"
-     xmlns="http://www.w3.org/2000/svg"
+     class="euiButtonIcon__icon"
+     data-euiicon-type="user"
    />

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation examples

• Added or updated jest tests
• Checked for breaking changes and labeled appropriately

- [ ] Checked for accessibility including keyboard-only and screenreader modes

• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2878/

[JoshMock]

That comment was a test that I ran locally. Probably not an accurate rendering of this PR's change. 😄

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2878/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2878/

[chandlerprall]

Snapshot updates look good. All tests pass and docs build.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2878/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2878/