C#: Add some string interpolation tests with alignment and formatting.

csharp/ql/test/library-tests/stringinterpolation/stringInterpolation.ql

@@ -0,0 +1,25 @@
+import csharp
+
+query predicate inserts(InterpolatedStringExpr expr, Expr e) {
+  expr.getAnInsert() = e // and inSpecificSource(expr)
+}
+
+query predicate texts(InterpolatedStringExpr expr, StringLiteral literal) {
+  expr.getAText() = literal // and inSpecificSource(expr)
+}
+
+query predicate interpolationInsertsWithAlign(InterpolatedStringExpr expr, Expr insert, Expr align) {
+  exists(InterpolatedStringInsertExpr e | expr.getInterpolatedInsert(_) = e |
+    insert = e.getInsert() and
+    align = e.getAlignment()
+  )
+}
+
+query predicate interpolationInsertsWithFormat(
+  InterpolatedStringExpr expr, Expr insert, StringLiteral format
+) {
+  exists(InterpolatedStringInsertExpr e | expr.getInterpolatedInsert(_) = e |
+    insert = e.getInsert() and
+    format = e.getFormat()
+  )
+}