Execute backtrace once before testing formatting (#38886)

On some platforms (PowerPC) the call to the `jlplt` is not a tail-call
and so it will be part of the backtrace. This means we are off-by-one
and won't skip the Julia function `backtrace` messing up tests that
check precise formatting.

(cherry picked from commit fc577d0)

stdlib/Logging/test/runtests.jl

@@ -193,6 +193,9 @@ end
  └ SUFFIX
  """
 
+ # Execute backtrace once before checking formatting, see #3885
+ backtrace()
+
  # Attaching backtraces
  bt = func1()
  @test startswith(genmsg("msg", exception=(DivideError(),bt)),

test/errorshow.jl

@@ -632,6 +632,8 @@ catch ex
 end
 pop!(Base.Experimental._hint_handlers[DomainError]) # order is undefined, don't copy this
 
+# Execute backtrace once before checking formatting, see #38858
+backtrace()
 
 # issue #28442
 @testset "Long stacktrace printing" begin