Fix #827 Pester TestName w/expandable str returns nothing (#851)

* Fix #827 Pester TestName w/expandable str returns nothing

This update will return null to indicate that the TestName arg was
present but not something we could evaluate.  The extension will see the
null value and pop a dialog box.

* Address PR feedback, simplify logic

Author: rkeithhill

src/PowerShellEditorServices/Symbols/PesterDocumentSymbolProvider.cs

@@ -32,8 +32,7 @@ IEnumerable<SymbolReference> IDocumentSymbolProvider.ProvideDocumentSymbols(
 
             return commandAsts.OfType<CommandAst>()
                               .Where(IsPesterCommand)
-                              .Select(ast => ConvertPesterAstToSymbolReference(scriptFile, ast))
-                              .Where(pesterSymbol => pesterSymbol?.TestName != null);
+                              .Select(ast => ConvertPesterAstToSymbolReference(scriptFile, ast));
         }
 
         /// <summary>
@@ -105,20 +104,21 @@ private static PesterSymbolReference ConvertPesterAstToSymbolReference(ScriptFil
                 // Check for an explicit "-Name" parameter
                 if (currentCommandElement is CommandParameterAst parameterAst)
                 {
+                    // Found -Name parameter, move to next element which is the argument for -TestName
                     i++;
-                    if (parameterAst.ParameterName == "Name" && i < pesterCommandAst.CommandElements.Count)
+
+                    if (!alreadySawName && TryGetTestNameArgument(pesterCommandAst.CommandElements[i], out testName))
                     {
-                        testName = alreadySawName ? null : (pesterCommandAst.CommandElements[i] as StringConstantExpressionAst)?.Value;
                         alreadySawName = true;
                     }
+
                     continue;
                 }
 
                 // Otherwise, if an argument is given with no parameter, we assume it's the name
                 // If we've already seen a name, we set the name to null
-                if (pesterCommandAst.CommandElements[i] is StringConstantExpressionAst testNameStrAst)
+                if (!alreadySawName && TryGetTestNameArgument(pesterCommandAst.CommandElements[i], out testName))
                 {
-                    testName = alreadySawName ? null : testNameStrAst.Value;
                     alreadySawName = true;
                 }
             }
@@ -131,6 +131,19 @@ private static PesterSymbolReference ConvertPesterAstToSymbolReference(ScriptFil
                 pesterCommandAst.Extent
             );
         }
+
+        private static bool TryGetTestNameArgument(CommandElementAst commandElementAst, out string testName)
+        {
+            testName = null;
+
+            if (commandElementAst is StringConstantExpressionAst testNameStrAst)
+            {
+                testName = testNameStrAst.Value;
+                return true;
+            }
+
+            return (commandElementAst is ExpandableStringExpressionAst);
+        }
     }
 
     /// <summary>