[LNT] Fix the graph page to show multiple metrics for the same test

Note: request.args is a dict and duplicate keys are ignored.
Currently the plot args have the following format plot.unused=m.t.f where unused is the test id.
If you select few metrics for the same test (for example compile_time and execution_time) on the Run page and click Graph, the URL will look like /db_default/v4/nts/graph?plot.958=5.958.2&plot.958=5.958.3
The second plot argument with the same key will be ignored. This patch fixes this issue.

OS Laboratory. Huawei Russian Research Institute. Saint-Petersburg

Reviewed By: cmatthews

Differential Revision: https://reviews.llvm.org/D118170

Author: Pavel Kosov

lnt/server/ui/templates/v4_run.html

@@ -345,7 +345,7 @@ <h4>Parameters</h4>
         <tbody class="searchable">
           {% for test_name,test_id,cr in tests %}
             <tr>
-              <td><input type="checkbox" name="plot.{{test_id}}" value="{{machine.id}}.{{test_id}}.{{field_index}}"/></td>
+              <td><input type="checkbox" name="plot.{{test_id}}.{{field_index}}" value="{{machine.id}}.{{test_id}}.{{field_index}}"/></td>
               <td class="benchmark-name">
                 <a href="{{graph_base}}&amp;plot.{{test_id}}={{ machine.id}}.{{test_id}}.{{field_index}}">
                   {{ test_name }}