Fix test for #8116.

This caused a failure on some 32bit builds due to floating point
(in)accuracy. But Complex is already strict in both its components, and
we really just want to know the code works, so an easier solution is to
simply use `seq` to force evaluation.
Signed-off-by: Austin Seipp <aseipp@pobox.com>

testsuite/tests/ghci/scripts/T8116.script

 :m + Data.Complex
-((-2) :+ 0)**(1.5 :+ 0)
+((-2) :+ 0)**(1.5 :+ 0) `seq` putStrLn "BOOM"

testsuite/tests/ghci/scripts/T8116.stdout

-(-5.195736337412959e-16) :+ (-2.82842712474619)
+BOOM