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

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.

The failing path is ChainRulesTestUtils/src/testers.jl at line 154; start by reproducing the shown test_scalar(simo, 3.0) case and inspect how the tuple output reaches one. Define an informative failure for multiple outputs, or confirm the supported behavior boundary, then run the existing test suite and add coverage for this case.