add first unit tests

the ci has always had a 'mvn test' step, but there was nothing to
test. add junit 6 and pin surefire 3.6.0, extract the os detection
into a testable fromOsName method, and cover the movement step math
- including the axis-aligned cases that used to produce infinite
steps and never converge.

pom.xml

@@ -31,6 +31,13 @@
             <artifactId>jnativehook</artifactId>
             <version>2.2.2</version>
         </dependency>
+
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>6.1.3</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <distributionManagement>
@@ -48,6 +55,12 @@
 
     <build>
         <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>3.6.0</version>
+            </plugin>
+
             <plugin>
                 <groupId>org.sonarsource.scanner.maven</groupId>
                 <artifactId>sonar-maven-plugin</artifactId>