How to run a single g++ testcase?

[Dan Kegel]

OK, I'm stumped.  I can run a single gcc testcase by using
the foo.exp=testname argument to runtest, but I can't seem
to make that work for g++'s tests.

Here's what I've tried (this is after running gcc's 'make check',
so site.exp is already there):

#!/bin/sh
set -x
TOP=`pwd`
TARGET=powerpc-750-linux-gnu
COMBO=gcc-3.3-glibc-2.2.5
PREFIX=/opt/cegl-2.0/$TARGET/$COMBO
export PATH=${PREFIX}/bin:$TOP/result/dejagnu/bin:${PATH}
cd build/$TARGET/$COMBO/build-gcc/gcc/testsuite

#DEJAGNU=$TOP/boards/master.exp runtest --tool=g++ --target=$TARGET
g++.exp=compat/abi
#DEJAGNU=$TOP/boards/master.exp runtest --tool=g++ --target=$TARGET
g++.exp=g++.dg/compat/abi/bitfield1.C
#DEJAGNU=$TOP/boards/master.exp runtest --tool=g++ --target=$TARGET
g++.exp=g++.dg
DEJAGNU=$TOP/boards/master.exp runtest --tool=g++ --target=$TARGET 

Only the last line, which doesn't try to specify a single test,
works.  Suggestions and examples welcome...

Thanks,
Dan