tests: subsys: logging: log_api: Fix test for 64 bit timestamp

Test for dropping log messages was expecting certain number of
dropped messages assuming that buffer size is dividable by the
message size. That was not the case when timestamp was 64 bit.
In that case, additional message is dropped.

Modified the test to take that into account.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>

tests/subsys/logging/log_api/src/test.inc

@@ -501,12 +501,24 @@ static void test_log_from_declared_module(void)
 	mock_log_backend_validate(&backend1, false);
 }
 
-static size_t get_short_msg_capacity(void)
+/** Calculate how many messages will fit in the buffer. Also calculate if
+ * remaining free space is size of message or not. This impacts how many messages
+ * are dropped. If free space is equal to message size then when buffer is full,
+ * adding new message will lead to one message drop, otherwise 2 message will
+ * be dropped.
+ */
+static size_t get_short_msg_capacity(bool *remainder)
 {
 	if (IS_ENABLED(CONFIG_LOG2)) {
-		return (CONFIG_LOG_BUFFER_SIZE / LOG2_SIMPLE_MSG_LEN) - 1;
+		*remainder = (CONFIG_LOG_BUFFER_SIZE % LOG2_SIMPLE_MSG_LEN) ?
+				true : false;
+
+		return (CONFIG_LOG_BUFFER_SIZE - sizeof(int)) / LOG2_SIMPLE_MSG_LEN;
 	}
 
+	*remainder = (CONFIG_LOG_BUFFER_SIZE % sizeof(struct log_msg)) ?
+			true : false;
+
 	return CONFIG_LOG_BUFFER_SIZE / sizeof(struct log_msg);
 }
 
@@ -552,13 +564,14 @@ static void test_log_msg_dropped_notification(void)
 		ztest_test_skip();
 	}
 
-	uint32_t capacity = get_short_msg_capacity();
+	bool remainder;
+	uint32_t capacity = get_short_msg_capacity(&remainder);
 
 	log_n_messages(capacity, 0);
 
 	/* Expect messages dropped when logger more than buffer capacity. */
-	log_n_messages(capacity + 1, 1);
-	log_n_messages(capacity + 2, 2);
+	log_n_messages(capacity + 1, 1 + (remainder ? 1 : 0));
+	log_n_messages(capacity + 2, 2 + (remainder ? 1 : 0));
 }
 
 /* Test checks if panic is correctly executed. On panic logger should flush all