Trying to add 2 floats an int and the char '-' to char array

Projects Programming
6

strcpy(x,y) replaces x with y. If you are trying to build on ling character array of sequential items then use strcat(x,y) char x[] will be appended with y[].

You didn't list the expected result, so I'm guessing from the information in the OP.

You expect to have
52.2831994.85105510-

as your output
That would be:
dtostrf(ss.getLat(),8,6,tmp);
strcat(testarray,tmp);
dtostrf(ss.getLng(),7,6,tmp);
strcat(testarray,tmp);
itoa(randominteger,tmp,10);
strcat(testarray,tmp);
strcat(testarray,"-");
Serial.print(testarray);

Or like Robin2 said just Serial.print() each item individually. The pi will receive the same data.