quote task_local_storage in docstrings (#39306)

Without the quoting, several terminals render the string as 
```
help?> Test.push_testset
  push_testset(ts::AbstractTestSet)

  Adds the test set to the tasklocalstorage.
```
with the word `local` being underlined.

stdlib/Test/src/Test.jl

@@ -1302,7 +1302,7 @@ end
 """
  push_testset(ts::AbstractTestSet)
 
-Adds the test set to the task_local_storage.
+Adds the test set to the `task_local_storage`.
 """
 function push_testset(ts::AbstractTestSet)
  testsets = get(task_local_storage(), :__BASETESTNEXT__, AbstractTestSet[])
@@ -1313,7 +1313,7 @@ end
 """
  pop_testset()
 
-Pops the last test set added to the task_local_storage. If there are no
+Pops the last test set added to the `task_local_storage`. If there are no
 active test sets, returns the fallback default test set.
 """
 function pop_testset()