New/Updated test cases. Fixed TraceTest so that it isn't causing test

New/Updated test cases.  Fixed TraceTest so that it isn't causing test
failures anymore.  Added SwitchesTest (to test switches) and
DiagnosticsConfigurationHandler to test .config file handling.

Issues: while the DiagnosticsConfigurationHandlerTest is reasonably
self-contained, SwitchesTest is not.  To properly test switches, we need a
.exe and a .exe.config file.  The needed .exe.config file that SwitchesTest
depends on is present in `test-config-file', but I'm unsure where to place it
(much less what to name it).

Additionally, SwitchesTest generates test failures at this time, principally
because of the lack of .config file support in the test system.

Finally, makefile.gnu & system_linux_test.args was changed to use the newly
added tests and to reference System.Xml.dll, which is used by
DiagnosticsConfigurationHandlerTest.

svn path=/trunk/mcs/; revision=9807