Add simple tests for PerUnit

wpiunits/src/test/java/edu/wpi/first/units/PerUnitTest.java

@@ -0,0 +1,56 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package edu.wpi.first.units;
+
+import static edu.wpi.first.units.Units.Degrees;
+import static edu.wpi.first.units.Units.Foot;
+import static edu.wpi.first.units.Units.Inches;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import org.junit.jupiter.api.Test;
+
+class PerUnitTest {
+  @Test
+  void ofNative() {
+    ExampleUnit part = new ExampleUnit(1);
+    var unit = PerUnit.combine(part, part);
+    assertEquals(10, unit.ofNative(10).in(unit));
+
+    // Does not compile:
+    // unit.of(10).in(unit);
+    //
+    // error: incompatible types: PerUnit<ExampleUnit,ExampleUnit> cannot be converted to CAP#1
+    //    unit.of(10).in(unit);
+    //                   ^
+    //  where CAP#1 is a fresh type-variable:
+    //    CAP#1 extends PerUnit<ExampleUnit,ExampleUnit>
+    //    from capture of ? extends PerUnit<ExampleUnit,ExampleUnit>
+    // This is because `of` returns a `Measure<? extends PerUnit<ExampleUnit, ExampleUnit>>`,
+    // and a `Per<ExampleUnit, ExampleUnit>` object isn't a subtype of that anonymous wildcard bound
+  }
+
+  @Test
+  void multSameDenom() {
+    var unit = Degrees.per(Foot);
+    var result = unit.mult(Foot);
+
+    // Multiplying by the same unit as the divisor should return the dividend unit
+    assertSame(Degrees, result);
+  }
+
+  @Test
+  void multOtherDenom() {
+    var unit = Degrees.per(Foot);
+
+    // (Degrees / Foot) x Inches, or 1/12 of a degree
+    var result = unit.mult(Inches);
+    assertInstanceOf(AngleUnit.class, result);
+    assertEquals(1 / 12.0, result.of(1).in(Degrees), 1e-9);
+    assertEquals("Degree per Foot Inch", result.name());
+    assertEquals("°/ft-in", result.symbol());
+  }
+}