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.

So: we can move it safely back, and it improves quality of life.

This reverts parts of commit d4630aa.

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>
@@ -512,7 +513,6 @@
 
                             <!-- Pass the following three directly to the JVM,
                                  because systemPropertyVariables are not passed correctly by Maven -->
-                            -Duser.timezone=${air.test.timezone}
                             -Duser.language=${air.test.language}
                             -Duser.region=${air.test.region}