Add tests for eltype(::Type{<:AbstractString}) (#58728)

Author: nilesh646

test/strings/basic.jl

@@ -1463,3 +1463,12 @@ if Sys.iswindows()
         @test Base.cwstring(str_3) == UInt16[0x0061, 0x0723, 0xd808, 0xdc00, 0x0000]
     end
 end
+
+
+@testset "eltype for AbstractString subtypes" begin
+    @test eltype(String) == Char
+    @test eltype(SubString{String}) == Char
+
+    u = b"hello"
+    @test eltype(u) === UInt8
+end