Fix ldexp test errors

[devmotion]

This PR fixes the test error caused by JuliaDiff/DiffRules.jl#73:

julia>] test ForwardDiff
...
...auto-testing Base.ldexp with 2 arguments
ERROR: LoadError: MethodError: no method matching ldexp(::Int64, ::Float64)
Closest candidates are:
  ldexp(::ForwardDiff.Dual{Tx}, ::AbstractFloat) where Tx at ~/.julia/packages/ForwardDiff/tZ5o1/src/dual.jl:144
  ldexp(::ForwardDiff.Dual{Tx}, ::Real) where Tx at ~/.julia/packages/ForwardDiff/tZ5o1/src/dual.jl:144
  ldexp(::Integer, ::ForwardDiff.Dual{Ty}) where Ty at ~/.julia/packages/ForwardDiff/tZ5o1/src/dual.jl:145
  ...
Stacktrace:
 [1] ldexp(x::ForwardDiff.Dual{Main.DualTest.TestTag(), Int64, 1}, y::Float64)
   @ ForwardDiff ~/.julia/packages/ForwardDiff/tZ5o1/src/dual.jl:243
...

[devmotion]

Random values where x/y where (close to) an integer caused problems in DiffRules (JuliaDiff/DiffRules.jl#78), so I thought it's good to apply the same fix here as well.

[codecov-commenter]

Codecov Report

Merging #568 (6268c6c) into master (ab0e239) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #568      +/-   ##
==========================================
+ Coverage   85.36%   85.38%   +0.01%     
==========================================
  Files           9        9              
  Lines         868      869       +1     
==========================================
+ Hits          741      742       +1     
  Misses        127      127              

Impacted Files	Coverage Δ
src/dual.jl	74.83% <0.00%> (+0.08%)	⬆️

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