Partially revert "Fix the way locale information is passed to Surefire"

The original commit was done 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. On the other hand,
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, and 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}