[lldb] [lldbsuite] Use a unique class name for TestValueVarUpdate

It looks like when this test was added, it was based on TestHelloWorld and it ended up with the same class name. This is an issue because the logs associated with the tests use the class name as the identifier for the file and if two tests have the same name their logs overwrite each other. On non-windows, this just means we lose one of the logs, but on Windows this means that one of the tests will fail occasionally because the file are locked by the other test.

llvm-svn: 364860