selftests: lib.mk: add TEST_CUSTOM_PROGS to allow custom test run/install

Some tests such as sync can't use generic build rules in lib.mk and require
custom rules. Currently there is no provision to allow custom builds and
test such as sync use TEST_PROGS which is reserved for test shell scripts.
Add TEST_CUSTOM_PROGS variable to lib.mk to run and install custom tests
built by individual test make files.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>