Tests asynchronous call stacks for DataTransferItem.getAsString.

Set timer for test function.
Captured call stacks in no particular order:
Call stack:
    0) onGetAsString (async-callstack-get-as-string.js:46)
    [DataTransferItem.getAsString]
    0) onPaste (async-callstack-get-as-string.js:41)
    1) timeout (async-callstack-get-as-string.js:32)
    [setTimeout]
    0) testFunction (async-callstack-get-as-string.js:17)
    [setTimeout]
    0) scheduleTestFunction <omitted>
    <... skipped remaining frames ...>

Call stack:
    0) onPaste (async-callstack-get-as-string.js:37)
    1) timeout (async-callstack-get-as-string.js:32)
    [setTimeout]
    0) testFunction (async-callstack-get-as-string.js:17)
    [setTimeout]
    0) scheduleTestFunction <omitted>
    <... skipped remaining frames ...>