(decedent allegedly owed debt to predeceased husband based on agreement that couple would keep their estates balanced; decedent's estate claimed deduction for such debt; court upheld IRS denial of deduction due to lack of evidence supporting debt; agreement was merely "friendly agreement between spouses" based on desire to minimize estate tax; agreement not entered into as result of business relationship; fact that debt recorded on both decedent's and spouse's estates' returns relatively minor fact compared to other evidence).