Add partitioningIndex(where:) unit tests where all elements in a collection belong in one of the two partitions

Author: mdznr

Tests/SwiftAlgorithmsTests/PartitionTests.swift

@@ -102,6 +102,16 @@ final class PartitionTests: XCTestCase {
     XCTAssertEqual(b, input.endIndex)
   }
 
+  func testPartitioningIndexWithOneEmptyPartition() {
+    let input: Range<Int> = (0 ..< 10)
+    
+    let a = input.partitioningIndex(where: { $0 > 10 })
+    XCTAssertEqual(a, input.endIndex)
+    
+    let b = input.partitioningIndex(where: { $0 >= 0 })
+    XCTAssertEqual(b, input.startIndex)
+  }
+  
   func testPartitionWithSubrangeBidirectionalCollection() {
     for length in 10...20 {
       let a = Array(0..<length)