Fannie Mae Whistle-Blower Won't Testify  WASHINGTON (Reuters) - A former Fannie Mae &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=FNM.N target=/stocks/quickinfo/fullquote"&gt;FNM.N&lt;/A&gt; employee  who gave U.S. officials information about what he saw as  accounting irregularities will not testify as planned before a  congressional hearing next week, a House committee said on  Friday.