Speed-up TestCase filtering on the TestPlan view page

by collecting a list of DOM selectors to be queried/shown via jQuery
multiple-selector at the same time instead of calling .show() for each
one of them.

For 1-2 rows this is on-par with the previous implementation, however it is
around 2x faster when the result set contains many rows (say 100 or
1000).

WARNING: when a result set contains hundreds of rows there is still a
noticeable lag on the page. For example if filtering 1000 rows for
is_automated=false, where only 10 rows have the value of True and the
rest 990 have the value of False!

See https://api.jquery.com/multiple-selector/

tcms/testplans/static/testplans/js/get.js

@@ -696,18 +696,25 @@ function filterTestCasesByProperty (planId, testCases, filterBy, filterValue) {
     }
 
     $('.js-testcase-row').hide()
+
+    // see https://api.jquery.com/multiple-selector/
+    const showOnly = []
     if (filterBy === 'component' || filterBy === 'tag') {
         const query = { plan: planId }
         query[`${filterBy}__name__icontains`] = filterValue
 
         jsonRPC('TestCase.filter', query, function (filtered) {
             // hide again if a previous async request showed something else
             $('.js-testcase-row').hide()
-            filtered.forEach(tc => $(`[data-testcase-pk=${tc.id}]`).show())
+            filtered.forEach(tc => showOnly.push(`[data-testcase-pk=${tc.id}]`))
+
+            $(showOnly.join(',')).show()
         })
     } else {
         testCases.filter(function (tc) {
             return (tc[filterBy] !== undefined && tc[filterBy].toString().toLowerCase().indexOf(filterValue) > -1)
-        }).forEach(tc => $(`[data-testcase-pk=${tc.id}]`).show())
+        }).forEach(tc => showOnly.push(`[data-testcase-pk=${tc.id}]`))
+
+        $(showOnly.join(',')).show()
     }
 }