Add context to errorshow except_str tests (#56199)

With this, the error message in
https://buildkite.com/julialang/julia-master/builds/41054#019294ca-e2c5-41f2-a897-e2959715f154
would become
```julia
Error in testset errorshow:
Test Failed at /home/jishnu/juliaPR/usr/share/julia/test/errorshow.jl:226
  Expression: typeof(err) === $(Expr(:escape, :MethodError))
   Evaluated: StackOverflowError === MethodError
     Context: expr = :(+())
```
Having the failing expression displayed makes it easier to locate the
source of the error.

test/errorshow.jl

@@ -215,14 +215,17 @@ Base.show_method_candidates(buf, try bad_vararg_decl("hello", 3) catch e e end)
 @test occursin("bad_vararg_decl(!Matched::$Int, ::Any...)", String(take!(buf)))
 
 macro except_str(expr, err_type)
+    source_info = __source__
     return quote
         let err = nothing
             try
                 $(esc(expr))
             catch err
             end
             err === nothing && error("expected failure, but no exception thrown")
-            @test typeof(err) === $(esc(err_type))
+            @testset let expr=$(repr(expr))
+                $(Expr(:macrocall, Symbol("@test"), source_info, :(typeof(err) === $(esc(err_type)))))
+            end
             buf = IOBuffer()
             showerror(buf, err)
             String(take!(buf))
@@ -231,6 +234,7 @@ macro except_str(expr, err_type)
 end
 
 macro except_strbt(expr, err_type)
+    source_info = __source__
     errmsg = "expected failure, but no exception thrown for $expr"
     return quote
         let err = nothing
@@ -239,7 +243,9 @@ macro except_strbt(expr, err_type)
             catch err
             end
             err === nothing && error($errmsg)
-            @test typeof(err) === $(esc(err_type))
+            @testset let expr=$(repr(expr))
+                $(Expr(:macrocall, Symbol("@test"), source_info, :(typeof(err) === $(esc(err_type)))))
+            end
             buf = IOBuffer()
             showerror(buf, err, catch_backtrace())
             String(take!(buf))
@@ -248,6 +254,7 @@ macro except_strbt(expr, err_type)
 end
 
 macro except_stackframe(expr, err_type)
+    source_info = __source__
     return quote
        let err = nothing
            local st
@@ -257,7 +264,9 @@ macro except_stackframe(expr, err_type)
                st = stacktrace(catch_backtrace())
            end
            err === nothing && error("expected failure, but no exception thrown")
-           @test typeof(err) === $(esc(err_type))
+           @testset let expr=$(repr(expr))
+               $(Expr(:macrocall, Symbol("@test"), source_info, :(typeof(err) === $(esc(err_type)))))
+           end
            sprint(show, st[1])
        end
     end