Add more tests

- Prefix greater than input's count
- Algorithm stability

Author: rakaramos

Tests/SwiftAlgorithmsTests/PartialSortTests.swift

@@ -114,4 +114,53 @@ final class PartialSortTests: XCTestCase {
       [1, 2, 3, 4, 7, 20, 70, 90, 100]
     )
   }
+  
+  func testSortedPrefixWithHugePrefix() {
+    XCTAssertEqual(
+      [4, 2, 1, 3].sortedPrefix(.max),
+      [1, 2, 3, 4]
+    )
+  }
+  
+  func testSortedPrfixStability() {
+    let array = ["a", "b", "b", "c", "a"]
+      .enumerated()
+      .map { ($0.element, $0.offset) }
+      .map(Something.init)
+    
+    let expected = [
+      Something("a", 0),
+      Something("a", 4),
+      Something("b", 1),
+      Something("b", 2),
+      Something("c", 3)
+    ]
+    
+    XCTAssertEqual(
+      array.sortedPrefix(5),
+      expected
+    )
+  }
+}
+
+private class Something: CustomDebugStringConvertible {
+  var debugDescription: String { "Something(\"\(value)\", \(position)\"" }
+  
+  let position: Int
+  let value: String
+
+  init(_ value: String, _ position: Int) {
+    self.value = value
+    self.position = position
+  }
+}
+
+extension Something: Comparable {
+  static func == (lhs: Something, rhs: Something) -> Bool {
+    lhs.value == rhs.value
+  }
+
+  static func < (lhs: Something, rhs: Something) -> Bool {
+    lhs.value < rhs.value
+  }
 }