tests: logging: Improve compatibility with 64-bit platforms

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

Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>
diff --git a/tests/subsys/logging/log_backend_uart/src/main.c b/tests/subsys/logging/log_backend_uart/src/main.c
index 84811d3..a1ad92f 100644
--- a/tests/subsys/logging/log_backend_uart/src/main.c
+++ b/tests/subsys/logging/log_backend_uart/src/main.c
@@ -68,7 +68,7 @@
 
 		tx_len = uart_emul_get_tx_data(fixture->dev[i], tx_content, sizeof(tx_content));
 		zassert_equal(tx_len, strlen(TEST_DATA),
-			      "%d: TX buffer length does not match. Expected %d, got %d",
+			      "%zu: TX buffer length does not match. Expected %zu, got %zu",
 			      i, strlen(TEST_DATA), tx_len);
 		zassert_mem_equal(tx_content, TEST_DATA, strlen(TEST_DATA));
 	}
diff --git a/tests/subsys/logging/log_core_additional/src/log_test.c b/tests/subsys/logging/log_core_additional/src/log_test.c
index 5dcc783..f01a9b0 100644
--- a/tests/subsys/logging/log_core_additional/src/log_test.c
+++ b/tests/subsys/logging/log_core_additional/src/log_test.c
@@ -300,7 +300,7 @@
 		}
 
 		zassert_equal(backend1_cb.total_logs, backend1_cb.counter,
-			      "Unexpected amount of messages received. %d",
+			      "Unexpected amount of messages received. %zu",
 			      backend1_cb.counter);
 	}
 }