Fix persistence threshold filter with UoM (#4881)

* Add tests with UoM conversion

For `PersistenceThresholdFilter` add a test case that fails when
using temperatures in °C and a threshold in Kelvin. The threshold
should ignore the offset (0 °C = 273,15 K).

Add another test with length to increase coverage.

Signed-off-by: Christoph Gehrke <cgdev@duck.com>

* Fix PersistenceThresholdFilter UoM conversion

Use `toUnitRelative()` instead of `toUnit()` for delta (the difference
of old and current state). In particular for temperatures an offset
of different units (like °C and K) should be ignored when considering
the delta.

Signed-off-by: Christoph Gehrke <cgdev@duck.com>

---------

Signed-off-by: Christoph Gehrke <cgdev@duck.com>

Author: ea21fd6

bundles/org.openhab.core.persistence/src/main/java/org/openhab/core/persistence/filter/PersistenceThresholdFilter.java

@@ -104,7 +104,7 @@ public boolean apply(Item item) {
                     }
                 } else if (!unit.isBlank()) {
                     // consider unit only if not relative threshold
-                    delta = delta.toUnit(unit);
+                    delta = delta.toUnitRelative(unit);
                     if (delta == null) {
                         throw new UnconvertibleException("");
                     }

bundles/org.openhab.core.persistence/src/test/java/org/openhab/core/persistence/filter/PersistenceThresholdFilterTest.java

@@ -102,6 +102,10 @@ private static Stream<Arguments> argumentProvider() {
                 // quantity type, below threshold, absolute, with unit
                 Arguments.of(new QuantityType<>("10 m"), new QuantityType<>("10.002 m"), "mm", false, false),
                 // quantity type, above threshold, absolute, with unit
+                Arguments.of(new QuantityType<>("1 ft"), new QuantityType<>("11 in"), "mm", false, true),
+                // quantity type, below threshold, absolute, with unit
+                Arguments.of(new QuantityType<>("0 °C"), new QuantityType<>("0 °C"), "K", false, false),
+                // quantity type, above threshold, absolute, with unit
                 Arguments.of(new QuantityType<>("-10 °C"), new QuantityType<>("5 °C"), "K", false, true));
     }