C#: Add const interpolated string test case.

Author: michaelnebel

csharp/ql/test/library-tests/csharp10/constInterpolatedString.expected

@@ -0,0 +1,6 @@
+inserts
+| ConstInterpolatedString.cs:6:38:6:53 | $"..." | ConstInterpolatedString.cs:6:41:6:45 | access to constant hello |
+| ConstInterpolatedString.cs:7:44:7:65 | $"..." | ConstInterpolatedString.cs:7:54:7:63 | access to constant helloWorld |
+texts
+| ConstInterpolatedString.cs:6:38:6:53 | $"..." | ConstInterpolatedString.cs:6:47:6:52 | " World" |
+| ConstInterpolatedString.cs:7:44:7:65 | $"..." | ConstInterpolatedString.cs:7:46:7:52 | "Really " |

csharp/ql/test/library-tests/csharp10/constInterpolatedString.ql

@@ -0,0 +1,7 @@
+import csharp
+
+query predicate inserts(InterpolatedStringExpr expr, Expr e) { expr.getAnInsert() = e }
+
+query predicate texts(InterpolatedStringExpr expr, StringLiteral literal) {
+  expr.getAText() = literal
+}