Add test for ethdebug output on interfaces

Author: clonker

test/cmdlineTests/ethdebug_on_interface/args

@@ -0,0 +1 @@
+--ethdebug --via-ir --pretty-json --json-indent 4

test/cmdlineTests/ethdebug_on_interface/input.sol

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: GPL-2.0
+pragma solidity >=0.0;
+
+interface C {
+    function f() external;
+}

test/cmdlineTests/ethdebug_on_interface/output

@@ -0,0 +1,19 @@
+======= Debug Data (ethdebug/format/info/resources) =======
+{
+    "compilation": {
+        "compiler": {
+            "name": "solc",
+            "version": "<VERSION REMOVED>"
+        },
+        "sources": [
+            {
+                "id": 0,
+                "path": "input.sol"
+            }
+        ]
+    }
+}
+
+======= input.sol:C =======
+Debug Data (ethdebug/format/program):
+null