CBL-5541 : Update vector search test per changes in v1.8

* Port the change from release/3.2 branch.
* Test 5, 6, 10, 11, 12, 13, 14, 16, 17, 18, 20, 22, 23, 24, 25 : Use centroids = 8.
* Test 5, 6, 7, 8, 9, 10, 11, 12, 14, 17, 18, 20, 22, 24, 25 : Verify that the index is trained.
* Test16 : Verify that the index is untrained.
* Test 20 : Add the step to create the query after the index was deleted.

Objective-C/Tests/CustomLogger.h

@@ -30,6 +30,8 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void) reset;
 
+- (BOOL) containsString: (NSString *)string;
+
 @end
 
 NS_ASSUME_NONNULL_END

Objective-C/Tests/CustomLogger.m

@@ -42,6 +42,15 @@ - (void) reset {
     [_lines removeAllObjects];
 }
 
+- (BOOL) containsString: (NSString *)string {
+    for (NSString* line in _lines) {
+        if ([line containsString: string]) {
+            return YES;
+        }
+    }
+    return NO;
+}
+
 - (void)logWithLevel: (CBLLogLevel)level domain: (CBLLogDomain)domain message: (NSString*)message {
     [_lines addObject: message];        
 }