testing/ostest: Fix uninitialized variable warning in wdog test

[hujun260]

Summary

Fix compiler warning about uninitialized variable in the watchdog test. The flags variable in wdtest_rand() was not initialized and could trigger -Werror=maybe-uninitialized error on certain architectures (e.g., x86_64/intel64) where the compiler detects potential use before assignment.
should merge before apache/nuttx#18135

Changes

File: testing/ostest/wdog.c

• Initialize flags variable to 0 at declaration
• Fixes build error on qemu-intel64/nsh_pci_smp configuration

Root cause:
The flags variable is conditionally assigned only when cnt % 2 is true, but the compiler cannot determine at compile time that the corresponding leave_critical_section(flags) will only be called when the variable was actually assigned.

// Before: uninitialized
irqstate_t flags;

// After: initialized
irqstate_t flags = 0;

[cederom]

Thank you @hujun260 :-)