Test that Joining_Group refines Joining_Type

[eggrobin]

Fix #437.

With this test, #435 fails with the following error:

[ERROR] Tests run: 10, Failures: 1, Errors: 0, Skipped: 1, Time elapsed: 2.19 s <<< FAILURE! - in org.unicode.propstest.TestProperties
[ERROR] org.unicode.propstest.TestProperties.TestJoiningGroupConsistency  Time elapsed: 0.03 s  <<< FAILURE!
org.opentest4j.AssertionFailedError:
U+062F (د) ARABIC LETTER DAL
and
U+10EC2 (𐻂) ARABIC LETTER DAL WITH TWO DOTS VERTICALLY BELOW
have different joining types but are in the same joining group (Dal)
 ==> expected: <Right_Joining> but was: <Dual_Joining>
        at org.unicode.propstest.TestProperties.lambda$1(TestProperties.java:229)
        at org.unicode.propstest.TestProperties.lambda$0(TestProperties.java:227)
        at org.unicode.propstest.TestProperties.TestJoiningGroupConsistency(TestProperties.java:223)

[markusicu]

lgtm

please squash-and-merge with a better commit message

FYI @roozbehp

[roozbehp]

I don't know all the internals, but the logic LGTM too.

[eggrobin]

better commit message

By default the squash-and-merge commit message is the name of the PR, so the https://xkcd.com/1296/ messages should not end up in the history :-)