Better error message when using `test_scalar` for functions with multiple outputs

[mzgubic]

When using test_scalar to test functions with multiple outputs the error messages are uninformative:

julia> simo(x) = (x, 2x)

julia> @scalar_rule simo(x) 1.0 2.0

julia> test_scalar(simo, 3.0)
Test Summary:                 | Pass  Total
simo at 3.0, with tangent 1.0 |    3      3
ERROR: MethodError: no method matching one(::Tuple{Float64,Float64})
Closest candidates are:
  one(::Type{Missing}) at missing.jl:103
  one(::BitArray{2}) at bitarray.jl:400
  one(::Missing) at missing.jl:100
  ...
Stacktrace:
 [1] #test_scalar#26(::Float64, ::Float64, ::FiniteDifferences.FiniteDifferenceMethod{Array{Int64,1},Array{Float64,1},FiniteDifferences.var"#27#29"{FiniteDifferences.FiniteDifferenceMethod{Array{Int64,1},Array{Float64,1},FiniteDifferences.var"#default_bound_estimator#19"{Int64}}}}, ::NamedTuple{(),Tuple{}}, ::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(test_scalar), ::typeof(simo), ::Float64) at /Users/mzgubic/Projects/ChainRules.jl/dev/ChainRulesTestUtils/src/testers.jl:154
 [2] test_scalar(::Function, ::Float64) at /Users/mzgubic/Projects/ChainRules.jl/dev/ChainRulesTestUtils/src/testers.jl:125
 [3] top-level scope at REPL[23]:1

We could also support multiple outputs though I don't think it is a priority given rrule_test and frule_test are available.