[SPARK-14749][SQL, TESTS] PlannerSuite failed when it run individually

## What changes were proposed in this pull request?

3 testcases namely,

```
"count is partially aggregated"
"count distinct is partially aggregated"
"mixed aggregates are partially aggregated"
```

were failing when running PlannerSuite individually.
The PR provides a fix for this.

## How was this patch tested?

unit tests

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: Subhobrata Dey <sbcd90@gmail.com>

Closes #12532 from sbcd90/plannersuitetestsfix.

sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala

@@ -51,7 +51,10 @@ class QueryExecution(val sqlContext: SQLContext, val logical: LogicalPlan) {
     }
   }
 
-  lazy val analyzed: LogicalPlan = sqlContext.sessionState.analyzer.execute(logical)
+  lazy val analyzed: LogicalPlan = {
+    SQLContext.setActive(sqlContext)
+    sqlContext.sessionState.analyzer.execute(logical)
+  }
 
   lazy val withCachedData: LogicalPlan = {
     assertAnalyzed()