let dg-test clear tool-specific test variables

[Janis Johnson]

Multiple places in the GCC testsuite support override dg-test from
DejaGnu in order to clear variables that are used by a tool's testsuite
framework.  The variables are cleared when an error is detected or after
the test has been run.  The overrides call dg-test from DejaGnu to do
most of the work.

Please provide a way (perhaps a tool-specific callback) for dg-test to
clear tool-specific test variables.