Add forkCount for surefire

Fixes following warnings in the build:
```
[WARNING] useSystemClassLoader setting has no effect when not forking
[WARNING] The parameter forkCount should likely not be 0. Forking a JVM
for tests improves test accuracy. Ensure to have a <forkCount> >= 1.
```

Author: akurtakov

tests/org.eclipse.swt.tests/pom.xml

@@ -25,6 +25,7 @@
   <properties>
     <tycho.testArgLine></tycho.testArgLine>
     <test>AllTests</test>
+    <forkCount>1</forkCount>
   </properties>
   <build>
     <plugins>