tests: logging: Improve compatibility with 64-bit platforms

[tpambor]

Use %zu format specifier for size_t type to ensure compatibility with both 32-bit and 64-bit platforms.

This fixes warnings when using zassert string validation, e.g.

/home/user/west_workspace/zephyr/tests/subsys/logging/log_backend_uart/src/main.c: In function 'log_backend_uart_test_log_backend_uart_multi_instance':
/home/user/west_workspace/zephyr/tests/subsys/logging/log_backend_uart/src/main.c:71:31: error: format '%d' expects argument of type 'int', but argument 7 has type 'size_t' {aka 'long unsigned int'} [-Werror=format=]
   71 |                               "%d: TX buffer length does not match. Expected %d, got %d",
      |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   72 |                               i, strlen(TEST_DATA), tx_len);
      |                               ~
      |                               |
      |                               size_t {aka long unsigned int}

Part of #96968

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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