test: added test cases for UI components

[sundasnoreen12]

Description
INF-917

Added unit tests for following notifications tray components.

1. Index.jsx
   a. Successfully showed bell icon and unseen count on it if unseen count is greater then 0
   b. Successfully showed notification tray by clicking on the bell icon
   c. Successfully hide notification tray by clicking on the bell icon
   d. Successfully showed setting icon in the notification tray

2. NotificationTabs.jsx

   1. Successfully showed list of notification tabs
   2. Successfully showed discussion tab selected by default
   3. Selected Tab has no unseen counts
   4. Successfully showed unseen counts for unselected tabs
   5. Successfully selected reminder tab

3. NotificationSection.jsx

   1. Successfully viewed two sections last 24 hours and earlier
   2. Successfully viewed mark all as read label on the UI
   3. Successfully viewed unread status across unread notification
   4. When the user clicks on the mark all as read, the unread status disappeared across all the notifications
   5. Successfully load more notifications by clicking on load more notification button

4. NotificationRowItem.jsx

   1. Successfully viewed notification icon, notification context, unread notification icon, course name and notification
      created time for each of the notification in both last 24 hours and earlier section
   2. Successfully marked notification as read

Tested
npm run test

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +16.76 🎉

Comparison is base (45a1da9) 62.39% compared to head (7229bff) 79.15%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #360       +/-   ##
===========================================
+ Coverage   62.39%   79.15%   +16.76%     
===========================================
  Files          30       31        +1     
  Lines         585      595       +10     
  Branches      108      108               
===========================================
+ Hits          365      471      +106     
+ Misses        212      121       -91     
+ Partials        8        3        -5     

Impacted Files	Coverage Δ
src/Notifications/NotificationRowItem.jsx	100.00% <ø> (+83.33%)	⬆️
src/Notifications/NotificationTabs.jsx	100.00% <ø> (+93.33%)	⬆️
src/Notifications/index.jsx	91.30% <ø> (+86.95%)	⬆️
src/Notifications/NotificationSections.jsx	100.00% <100.00%> (+95.65%)	⬆️
src/Notifications/test-utils.js	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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