Realtime tests: Fix robust mutex conditionals

sbrk_mutex and testpi-6 realtime tests in subdir
testcases/realtime/func/pi-tests used compile time config variables
which were not generated by autoconf in order to configure tests
for robust mutexes.  Changed these conditionals to use the config
variables actually created in the autoconf process.

Signed-off-by: Gary S. Robertson <gary.robertson@linaro.org>
Acked-by: Jan Stancek <jstancek@redhat.com>
Realtime tests: Use correct PI conditional in testpi-5

testcases/realtime/func/pi-tests/testpi-5.c
referenced a non-existent autoconf configuration setting variable.
Replaced the invalid variable name with the variable actually created
by autoconf.

Signed-off-by: Gary S. Robertson <gary.robertson@linaro.org>
Acked-by: Jan Stancek <jstancek@redhat.com>