Table collision in join.sql and aggregates.sql

As we've been merging our code with 9.6, a couple developers have had
one-off failures in the join.sql and aggregates.sql test because the tables
T1, T2 and T3 have the wrong definitions.

Digging into it, I found that both files create the tables T1, T2, and T3
for a short period of time and then drop them. Since the parallel_schedule
file puts these two files into the same group, they can run concurrently.
If it happens that the the two files hit the T1, T2, T3 tests at the same
time, then you see the table definition problems.

I took the easy way of solving this and renamed the tables in
aggregates.sql to AGG1, AGG2, and AGG3. (Picked on aggregates.sql since it
had the T1, T2, T3 tests added in 9.6.)

Doug
- Salesforce