libstdc++/5145: [cygwin] Failure of test 23_containers/map_operators.cc

[Benjamin Kosnik]

both libstdc++/5145 and libstdc++/5146 are the same issue.

These tests should fail, if they don't it's an error. You'd think that
this is ust a question of doing dg-error on the line in question, but
it doesn't seem tow work with multi-line C++ errors or something, so
the "excess errors" part was put in.

If you can think of a better way to do this, and or one that
eliminates the excess errors markings in these two files I'm all ears.

-benjamin