Add tests for CompositeException (#54174)

Bunch of methods appear untested on Coveralls:
https://coveralls.io/builds/67039158/source?filename=base%2Ftask.jl#L46

test/error.jl

@@ -139,3 +139,24 @@ end
 using .Issue54029
 @test Base.infer_return_type(raise54029, (Any,)) === Union{}
 @test Base.infer_return_type(xs->raise54029(xs...), (Vector{Any},)) === Union{}
+
+@testset "CompositeException" begin
+    ce = CompositeException()
+    @test isempty(ce)
+    @test length(ce) == 0
+    @test eltype(ce) == Any
+    str = sprint(showerror, ce)
+    @test str == "CompositeException()\n"
+    push!(ce, ErrorException("something sad has happened"))
+    @test !isempty(ce)
+    @test length(ce) == 1
+    pushfirst!(ce, ErrorException("something sad has happened even earlier"))
+    @test length(ce) == 2
+    # test iterate
+    for ex in ce
+        @test ex isa ErrorException
+    end
+    push!(ce, ErrorException("something sad has happened yet again"))
+    str = sprint(showerror, ce)
+    @test str == "something sad has happened even earlier\n\n...and 2 more exceptions.\n"
+end