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.

src/test/java/wtf/beatrice/autosqueal/util/SystemUtilTest.java

@@ -0,0 +1,34 @@
+package wtf.beatrice.autosqueal.util;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class SystemUtilTest
+{
+    @Test
+    void detectsWindows() {
+        assertEquals(SystemUtil.OperatingSystem.WINDOWS, SystemUtil.fromOsName("Windows 11"));
+    }
+
+    @Test
+    void detectsLinux() {
+        assertEquals(SystemUtil.OperatingSystem.LINUX, SystemUtil.fromOsName("Linux"));
+        assertEquals(SystemUtil.OperatingSystem.LINUX, SystemUtil.fromOsName("GNU/Linux"));
+    }
+
+    @Test
+    void detectsMacOS() {
+        assertEquals(SystemUtil.OperatingSystem.MAC_OS, SystemUtil.fromOsName("Mac OS X"));
+    }
+
+    @Test
+    void unknownForOtherSystems() {
+        assertEquals(SystemUtil.OperatingSystem.UNKNOWN, SystemUtil.fromOsName("SunOS"));
+    }
+
+    @Test
+    void hostSystemMatchesOsName() {
+        assertEquals(SystemUtil.fromOsName(System.getProperty("os.name")), SystemUtil.getHostSystem());
+    }
+}