make test name cleanly available to test support for tool

[Janis Johnson]

In several places the GCC testsuite needs to know the name of the
current test.  Sometimes it uses it to report unresolved test steps and
sometimes it uses it to construct the name of a generated file, such as
the assembly file or a debugging dump.

GCC gets the name of the current test by using upvar to access local
variable 'name' in dg-test, which is 1, 2, or 3 calls away from the
procedure that needs it.  Many of these accesses have a comment like

    # This assumes that we are two frames down from dg-test or some
    # other proc that stores the filename of the testcase in a local
    # variable "name".  A cleaner solution would require a new
    # DejaGnu release.

If DejaGnu makes available the test name in a cleaner way, such as a
global variable or a procedure call, then when that release is widely
available we can clean up the GCC testsuite support a bit.

Please make available the name of the current test.

Janis Johnson