Fix FwC test task registration

With this change it takes into account that not all versions have
previous minor unreleased version (because we are on the oldest active
development branch), or that current version has minor == 0 (e.g. 9.0)
so previous minor have to be calculated differently.

Author: jozala

build-tools-internal/src/main/groovy/elasticsearch.fwc-test.gradle

@@ -7,19 +7,23 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-import org.elasticsearch.gradle.VersionProperties
+import org.elasticsearch.gradle.Version
 import org.elasticsearch.gradle.testclusters.StandaloneRestIntegTestTask
 
-def fwcVersions = buildParams.bwcVersions.released.findAll { it.major == VersionProperties.elasticsearchVersion.major &&  it.minor == VersionProperties.elasticsearchVersion.minor }
-def previousMinorSnapshot = buildParams.bwcVersions.unreleased.find { it.major == VersionProperties.elasticsearchVersion.major && it.minor == VersionProperties.elasticsearchVersion.minor - 1 }
-
-fwcVersions.each { fwcVersion ->
-  tasks.register("v${fwcVersion}#fwcTest", StandaloneRestIntegTestTask) {
-    usesBwcDistribution(previousMinorSnapshot)
-    usesBwcDistribution(fwcVersion)
-    systemProperty("tests.old_cluster_version", previousMinorSnapshot)
-    systemProperty("tests.new_cluster_version", fwcVersion)
-    nonInputProperties.systemProperty 'tests.fwc', 'true'
+Version elasticsearchVersion = Version.fromString(versions.get("elasticsearch"))
+def fwcVersions = buildParams.bwcVersions.released.findAll { it.major == elasticsearchVersion.major &&  it.minor == elasticsearchVersion.minor }
+def targetMajor = elasticsearchVersion.minor > 0 ? elasticsearchVersion.major : elasticsearchVersion.major - 1
+def targetMinor = elasticsearchVersion.minor > 0 ? elasticsearchVersion.minor - 1 : buildParams.bwcVersions.unreleased.findAll { it.major == targetMajor }*.minor.max()
+def previousMinorSnapshot = buildParams.bwcVersions.unreleased.find { it.major == targetMajor && it.minor == targetMinor }
+if (previousMinorSnapshot != null) {
+  fwcVersions.each { fwcVersion ->
+    tasks.register("v${fwcVersion}#fwcTest", StandaloneRestIntegTestTask) {
+      usesBwcDistribution(previousMinorSnapshot)
+      usesBwcDistribution(fwcVersion)
+      systemProperty("tests.old_cluster_version", previousMinorSnapshot)
+      systemProperty("tests.new_cluster_version", fwcVersion)
+      nonInputProperties.systemProperty 'tests.fwc', 'true'
+    }
   }
 }