Fix running timezone-sensitive tests in IntelliJ IDEA

Some time ago, some properties were moved to `argLine` because Surefire
incorrectly passes the `user.language` and `user.region` system
properties to the tests. At that time, the `user.timezone` also got
along for the ride, but it turns out, that that latter property *is*
passed correctly. So while it works for Maven and Surefire, moving it
into `argLine` causes IntelliJ IDEA to not pick it up, which in turn
means that some tests are not working properly when executed inside the
IDE. We can move it safely back, as it improves quality of life.

This reverts parts of commit d4630aa.

CHANGES.md

@@ -1,3 +1,9 @@
+Airbase 140
+
+* Pass `user.timezone` in tests as a system property, not as a JVM argument
+  - It restores the ability to run some timezone-sensitive tests
+    from within IntelliJ IDEA
+
 Airbase 139
 
 * Replace checkstyle illegal imports with `RestrictImports` enforcer rules

airbase/pom.xml

@@ -498,6 +498,7 @@
                     <configuration>
                         <systemPropertyVariables>
                             <sun.jnu.encoding>${project.build.sourceEncoding}</sun.jnu.encoding>
+                            <user.timezone>${air.test.timezone}</user.timezone>
                             <java.awt.headless>true</java.awt.headless>
                             <java.util.logging.SimpleFormatter.format>%1$tY-%1$tm-%1$tdT%1$tH:%1$tM:%1$tS.%1$tL%1$tz %4$s %5$s%6$s%n</java.util.logging.SimpleFormatter.format>
                         </systemPropertyVariables>
@@ -510,9 +511,7 @@
                         <argLine>
                             @{argLine}
 
-                            <!-- Pass the following three directly to the JVM,
-                                 because systemPropertyVariables are not passed correctly by Maven -->
-                            -Duser.timezone=${air.test.timezone}
+                            <!-- Pass the following properties directly to the JVM, because they are not passed correctly by Surefire: -->
                             -Duser.language=${air.test.language}
                             -Duser.region=${air.test.region}