Add test coverage for negative-day adjustment in DurationFormatUtils

Adds unit tests covering month-boundary and leap-year cases that
trigger the internal negative-days borrowing logic in
DurationFormatUtils.formatPeriod.

Removes obsolete TODO about missing JaCoCo coverage.

Author: nine0703

src/test/java/org/apache/commons/lang3/time/DurationFormatUtilsTest.java

@@ -473,6 +473,53 @@ void testFormatPeriod() {
         assertEquals("helloworld", DurationFormatUtils.formatPeriod(time1970, time, "'hello''world'"));
     }
 
+    /** Covers negative-day borrowing across month boundary */
+    @Test
+    void testFormatPeriodISONegativeDaysBorrowAcrossMonth() {
+        final TimeZone tz = TimeZones.getTimeZone("GMT-3");
+
+        final Calendar start = Calendar.getInstance(tz);
+        start.set(2020, Calendar.JANUARY, 31, 0, 0, 0);
+        start.set(Calendar.MILLISECOND, 0);
+
+        final Calendar end = Calendar.getInstance(tz);
+        end.set(2020, Calendar.FEBRUARY, 1, 0, 0, 0);
+        end.set(Calendar.MILLISECOND, 0);
+
+        final String result = DurationFormatUtils.formatPeriod(
+                start.getTimeInMillis(),
+                end.getTimeInMillis(),
+                DurationFormatUtils.ISO_EXTENDED_FORMAT_PATTERN,
+                false,
+                tz
+        );
+
+        assertEquals("P0Y0M1DT0H0M0.000S", result);
+    }
+
+    @Test
+    void testFormatPeriodISONegativeDaysLeapYear() {
+        final TimeZone tz = TimeZones.getTimeZone("GMT-3");
+
+        final Calendar start = Calendar.getInstance(tz);
+        start.set(2020, Calendar.FEBRUARY, 29, 0, 0, 0);
+        start.set(Calendar.MILLISECOND, 0);
+
+        final Calendar end = Calendar.getInstance(tz);
+        end.set(2020, Calendar.MARCH, 1, 0, 0, 0);
+        end.set(Calendar.MILLISECOND, 0);
+
+        final String result = DurationFormatUtils.formatPeriod(
+                start.getTimeInMillis(),
+                end.getTimeInMillis(),
+                DurationFormatUtils.ISO_EXTENDED_FORMAT_PATTERN,
+                false,
+                tz
+        );
+
+        assertEquals("P0Y0M1DT0H0M0.000S", result);
+    }
+
     @Test
     void testFormatPeriodeStartGreaterEnd() {
         assertIllegalArgumentException(() -> DurationFormatUtils.formatPeriod(5000, 2500, "yy/MM"));
@@ -505,8 +552,6 @@ void testFormatPeriodISO() {
         assertEquals("P1Y1M2DT10H30M0.000S", text);
         // want a way to say 'don't print the seconds in format()' or other fields for that matter:
         // assertEquals("P1Y2M3DT10H30M", text);
-        //
-        // TODO Jacoco shows missing coverage for internal negative days
     }
 
     @Test