pkg/tsdb/*: Fix do not pass a nil Context

See,
$ gometalinter --vendor --deadline 6m --disable-all --enable=megacheck ./...
pkg/tsdb/mssql/mssql_test.go:695:33:warning: do not pass a nil Context, even if a function permits it; pass context.TODO if you are unsure about which Context to use (SA1012) (megacheck)
pkg/tsdb/mysql/mysql_test.go:772:32:warning: do not pass a nil Context, even if a function permits it; pass context.TODO if you are unsure about which Context to use (SA1012) (megacheck)
pkg/tsdb/postgres/postgres_test.go:704:33:warning: do not pass a nil Context, even if a function permits it; pass context.TODO if you are unsure about which Context to use (SA1012) (megacheck)

pkg/tsdb/mssql/mssql_test.go

@@ -692,7 +692,7 @@ func TestMSSQL(t *testing.T) {
 					},
 				}
 
-				resp, err := endpoint.Query(nil, nil, query)
+				resp, err := endpoint.Query(context.Background(), nil, query)
 				So(err, ShouldBeNil)
 				queryResult := resp.Results["A"]
 				So(queryResult.Error, ShouldBeNil)

pkg/tsdb/mysql/mysql_test.go

@@ -769,7 +769,7 @@ func TestMySQL(t *testing.T) {
 				},
 			}
 
-			resp, err := endpoint.Query(nil, nil, query)
+			resp, err := endpoint.Query(context.Background(), nil, query)
 			So(err, ShouldBeNil)
 			queryResult := resp.Results["A"]
 			So(queryResult.Error, ShouldBeNil)

pkg/tsdb/postgres/postgres_test.go

@@ -701,7 +701,7 @@ func TestPostgres(t *testing.T) {
 					},
 				}
 
-				resp, err := endpoint.Query(nil, nil, query)
+				resp, err := endpoint.Query(context.Background(), nil, query)
 				So(err, ShouldBeNil)
 				queryResult := resp.Results["A"]
 				So(queryResult.Error, ShouldBeNil)