Fix WAIT_FOR_DEBUGGER for container tests.

For container tests, gdb needs to set the sysroot to the corresponding
testroot.root directory.  The assumption was that PIDs < 3 means that
we are running within a container.

Starting with commit 2fe64148
"Allow for unpriviledged nested containers", the default is to use
the PID namespace of the parent.  Thus support_test_main.c does not
recognize our container anymore.

This patch now assumes that we are running inside a container if
test-container.c has set PID_OUTSIDE_CONTAINER and always uses this
PID independent of having a new PID namespace or not.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>