tc-testing: Add tdc to kselftests

tc-testing: Add tdc to kselftests

Add tdc to existing kselftest infrastructure so that it can be run with
existing kselftests. TDC now generates objects in objdir/kselftest
without cluttering main objdir, leaves source directory clean, and
installs correctly in kselftest_install, properly adding itself to
run_kselftest.sh script.

Add tc-testing as a target of selftests/Makefile. Create tdc.sh to run
tdc.py targets with correct arguments. To support single target from
selftest/Makefile, combine tc-testing/bpf/Makefile and
tc-testing/Makefile. Move action.c up a directory to tc-testing/.

Tested with:
 make O=/tmp/{objdir} TARGETS="tc-testing" kselftest
 cd /tmp/{objdir}
 cd kselftest
 cd tc-testing
 ./tdc.sh

 make -C tools/testing/selftests/ TARGETS=tc-testing run_tests

 make TARGETS="tc-testing" kselftest
 cd tools/testing/selftests
 ./kselftest_install.sh /tmp/exampledir
 My VM doesn't run all the kselftests so I commented out all except my
 target and net/pmtu.sh then:
 cd /tmp/exampledir && ./run_kselftest.sh

Co-developed-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Briana Oursler <briana.oursler@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>