Explore: Remove datasource testing on selector (#19910)

* Explore: Remove datasource testing on selector

- datasource testing gets in the way of fast query iteration: switching
between datasources can take seconds
- it should not be explore's duty to test datasources in the first place
- removed the concept of datasourceError in Explore, should not be its
concern
- datasource erorrs will express themselves in query errors just fine
- connection errors are still bubbled up
- removed reconnection logic from explore, should not be its concern
- missing labels in loki are still "visible" via an empty label selector
- Loki and Prometheus treated connection errors differently than other
datasources, making sure to pass through the original error message

* Show datasource error in query field for prom/loki/influx

* Removed connection test case, fixed disabled state

public/app/plugins/datasource/prometheus/components/PromQueryEditor.tsx

@@ -2,7 +2,7 @@ import _ from 'lodash';
 import React, { PureComponent } from 'react';
 
 // Types
-import { FormLabel, Select, Switch, QueryEditorProps, DataSourceStatus } from '@grafana/ui';
+import { FormLabel, Select, Switch, QueryEditorProps } from '@grafana/ui';
 import { SelectableValue } from '@grafana/data';
 
 import { PrometheusDatasource } from '../datasource';
@@ -104,7 +104,6 @@ export class PromQueryEditor extends PureComponent<Props, State> {
           onChange={this.onFieldChange}
           history={[]}
           data={data}
-          datasourceStatus={DataSourceStatus.Connected} // TODO: replace with real DataSourceStatus
         />
 
         <div className="gf-form-inline">