java tests: fix path resolution and BigDecimal value comparison

Pass the Gradle project directory to tests as the ladybug.projectDir
system property so TestHelper can build absolute paths to dataset files
regardless of the working directory at test time.

ValueTest.testCreateValue: BigDecimal comparison now uses compareTo()
instead of equals() to handle scale differences (e.g. 1.5 vs 1.50).
Also fix two places in the map test loop that reused a stale LbugMap
reference instead of constructing a fresh one from the current tuple.