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.html:38)
    [DataTransferItem.getAsString]
    0) onPaste (async-callstack-get-as-string.html:33)
    1) timeout (async-callstack-get-as-string.html:24)
    [setTimeout]
    0) testFunction (async-callstack-get-as-string.html:9)

Call stack:
    0) onPaste (async-callstack-get-as-string.html:29)
    1) timeout (async-callstack-get-as-string.html:24)
    [setTimeout]
    0) testFunction (async-callstack-get-as-string.html:9)