Add a new isRHEL9 method to determine RHEL9 system.

Note: TBH - I'm not sure whether the `forceRhel9` method is still
necessary - I created it since this is what it is used for RHEL7 and
RHEL8 already.

Author: jstourac

core/src/main/groovy/noe/common/utils/Platform.groovy

@@ -132,6 +132,10 @@ class Platform {
     return (isRHEL() && (osVersion ==~ /.*el8.*/)) || forceRhel8()
   }
 
+  boolean isRHEL9() {
+    return (isRHEL() && (osVersion ==~ /.*el9.*/)) || forceRhel9()
+  }
+
   boolean isSolaris11() {
     return isSolaris() && (osVersion ==~ /5\.11/)
   }
@@ -186,4 +190,13 @@ class Platform {
   private boolean forceRhel8() {
     return Boolean.parseBoolean(Library.getUniversalProperty('force.rhel.8', 'false'))
   }
+
+  /**
+   * As on Docker, the 'os.name' of the system is determined by the host OS and not the container one, to make tests
+   * work it is necessary to mock that we are, indeed, using RHEL9 machine.
+   * @return value of the 'force.rhel.9' property
+   */
+  private boolean forceRhel9() {
+    return Boolean.parseBoolean(Library.getUniversalProperty('force.rhel.9', 'false'))
+  }
 }