test: Add edge case tests for YearsBeforeDesiredBalance (#2443)

Author: KaranChadha10

exercises/concept/interest-is-interesting/.meta/config.json

@@ -1,7 +1,8 @@
 {
   "authors": [
     "ErikSchierboom",
-    "yzAlvin"
+    "yzAlvin",
+    "karanchadha10"
   ],
   "files": {
     "solution": [

exercises/concept/interest-is-interesting/InterestIsInterestingTests.cs

@@ -197,4 +197,18 @@ public void Years_before_desired_balance_for_large_different_between_start_and_t
     {
         Assert.Equal(85, SavingsAccount.YearsBeforeDesiredBalance(2_345.67m, 12_345.6789m));
     }
+
+    [Fact]
+    [Task(4)]
+    public void Years_before_desired_balance_when_target_already_met()
+    {
+        Assert.Equal(0, SavingsAccount.YearsBeforeDesiredBalance(1_000.0m, 1_000.0m));
+    }
+
+    [Fact]
+    [Task(4)]
+    public void Years_before_desired_balance_when_target_already_exceeded()
+    {
+        Assert.Equal(0, SavingsAccount.YearsBeforeDesiredBalance(1_200.0m, 1_000.0m));
+    }
 }