selftests: lib.mk: suppress "cd" output from run_tests target

Suppress "cd" output from run_tests while running tests to declutter the
test results.

Running efivarfs test:
make --silent -C tools/testing/selftests/efivarfs/ run_tests

Before the change:

skip all tests: must be run as root
selftests: efivarfs.sh [PASS]
/lkml/linux-kselftest/tools/testing/selftests/efivarfs

After the change:

skip all tests: must be run as root
selftests: efivarfs.sh [PASS]

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