Fix Ledger test so they pass regardless of system locale

exercises/practice/ledger/.meta/config.json

@@ -10,7 +10,8 @@
       "src/main/java/Ledger.java"
     ],
     "test": [
-      "src/test/java/LedgerTest.java"
+      "src/test/java/LedgerTest.java",
+      "src/test/java/LocaleExtension.java"
     ],
     "example": [
       ".meta/src/reference/java/Ledger.java"

exercises/practice/ledger/src/test/java/LedgerTest.java

@@ -1,9 +1,11 @@
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Disabled;
-import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
+@ExtendWith(LocaleExtension.class)
 public class LedgerTest {
 
     private static final String US_LOCALE = "en-US";

exercises/practice/ledger/src/test/java/LocaleExtension.java

@@ -0,0 +1,21 @@
+import org.junit.jupiter.api.extension.AfterTestExecutionCallback;
+import org.junit.jupiter.api.extension.BeforeTestExecutionCallback;
+import org.junit.jupiter.api.extension.ExtensionContext;
+
+import java.util.Locale;
+
+public class LocaleExtension implements BeforeTestExecutionCallback, AfterTestExecutionCallback {
+    private Locale originalLocale;
+
+    @Override
+    public void beforeTestExecution(ExtensionContext context) throws Exception {
+        originalLocale = Locale.getDefault();
+
+        Locale.setDefault(Locale.US);
+    }
+
+    @Override
+    public void afterTestExecution(ExtensionContext context) throws Exception {
+        Locale.setDefault(originalLocale);
+    }
+}