Following documentation from @Kevin's answer the below one worked for me for setting environment variable with maven sure-fire plugin
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<configuration>
<environmentVariables>
<WSNSHELL_HOME>conf</WSNSHELL_HOME>
</environmentVariables>
</configuration>