test: Show cluster logs on failure for 21million systest. (#5507)

test-21million.sh can show failures like the following in CI builds:

```
------- Stdout: -------
=== RUN   TestQueries/query-044
    --- FAIL: TestQueries/query-044 (4.74s)
        run_test.go:86: 
            	Error Trace:	run_test.go:86
            	Error:      	Received unexpected error:
            	            	rpc error: code = Unknown desc = : rpc error: code = Unavailable desc = transport is closing
            	Test:       	TestQueries/query-044
```
```
------- Stdout: -------
=== RUN   TestQueries/query-045
    --- FAIL: TestQueries/query-045 (0.01s)
        run_test.go:86: 
            	Error Trace:	run_test.go:86
            	Error:      	Received unexpected error:
            	            	rpc error: code = Unknown desc = : rpc error: code = Unavailable desc = all SubConns are in TransientFailure, latest connection error: connection error: desc = "transport: Error while dialing dial tcp 172.24.0.4:7182: connect: connection refused"
            	Test:       	TestQueries/query-045
```

It looks like something happened to the Dgraph cluster, but we're
lost without any cluster logs.

This change prints out the cluster logs for alpha1, alpha2,
alpha3, and zero1 if there are test failures in
test-21million.sh.

Changes
* Print out cluster logs on test failure
* Move cluster tear down to the very end of the test so logs can
  be captured beforehand.

systest/21million/test-21million.sh

@@ -144,6 +144,20 @@ if [[ ! -z "$TEAMCITY_VERSION" ]]; then
 fi
 go test -v -tags standalone $SAVEDIR $QUIET || FOUND_DIFFS=1
 
+if [[ $FOUND_DIFFS -eq 0 ]]; then
+    Info "no diffs found in query results"
+else
+    Info "Cluster logs for alpha1"
+    docker logs alpha1
+    Info "Cluster logs for alpha2"
+    docker logs alpha2
+    Info "Cluster logs for alpha3"
+    docker logs alpha3
+    Info "Cluster logs for zero1"
+    docker logs zero1
+    Info "found some diffs in query results"
+fi
+
 if [[ $CLEANUP == all ]]; then
     Info "bringing down zero and alpha and data volumes"
     DockerCompose down -v
@@ -154,10 +168,4 @@ else
     DockerCompose down
 fi
 
-if [[ $FOUND_DIFFS -eq 0 ]]; then
-    Info "no diffs found in query results"
-else
-    Info "found some diffs in query results"
-fi
-
 exit $FOUND_DIFFS