tests: kernel: thread_apis: Fix -Wsometimes-uninitialized warning

Building kernel.threads.apis with clang warns:

tests/kernel/threads/thread_apis/src/main.c:362:6: error: variable 'ret'
is used uninitialized whenever 'if' condition is true
[-Werror,-Wsometimes-uninitialized]
        if (m == ISR_ALREADY_EXIT || m == ISR_RUNNING) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tests/kernel/threads/thread_apis/src/main.c:380:6: note: uninitialized
use occurs here
        if (ret != 0) {
            ^~~
tests/kernel/threads/thread_apis/src/main.c:362:2: note: remove the 'if'
if its condition is always false
        if (m == ISR_ALREADY_EXIT || m == ISR_RUNNING) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tests/kernel/threads/thread_apis/src/main.c:362:6: error: variable 'ret'
is used uninitialized whenever '||' condition is true
[-Werror,-Wsometimes-uninitialized]
        if (m == ISR_ALREADY_EXIT || m == ISR_RUNNING) {
            ^~~~~~~~~~~~~~~~~~~~~
tests/kernel/threads/thread_apis/src/main.c:380:6: note: uninitialized
use occurs here
        if (ret != 0) {
            ^~~
tests/kernel/threads/thread_apis/src/main.c:362:6: note: remove the '||'
if its condition is always false
        if (m == ISR_ALREADY_EXIT || m == ISR_RUNNING) {
            ^~~~~~~~~~~~~~~~~~~~~~~~
tests/kernel/threads/thread_apis/src/main.c:329:9: note: initialize the
variable 'ret' to silence this warning
        int ret;
               ^
                = 0

Signed-off-by: Tom Hughes <tomhughes@chromium.org>