[stdlib] Update tests for generic floating-point conversion

Author: xwu

test/stdlib/FloatingPoint.swift.gyb

@@ -206,6 +206,16 @@ FloatingPoint.test("BinaryFloatingPoint/genericFloatingPointConversion") {
     Float._convert(
       from: Double._convert(from: Float.leastNonzeroMagnitude).value).value,
     Float.leastNonzeroMagnitude)
+
+  // Let's make sure that the correct value is returned when two representable
+  // values are equally close to the original value.
+  let bitPattern: UInt64 = 0b01111111111_0000000000000000000000110000000000000000000000000000
+  var z = Double(bitPattern: bitPattern)
+  expectEqual(Float._convert(from: z).value, Float(z))
+
+  z = Double(Float.greatestFiniteMagnitude) +
+      Double(Float.greatestFiniteMagnitude.ulp / 2)
+  expectEqual(Float._convert(from: z).value, Float(z))
 }
 
 func positiveOne<T: ExpressibleByIntegerLiteral>() -> T {