Allow version 0.0.0 for tests

Need to keep allowing that for serverless tests that doesn't know the
version for the detached version distribution when registering the BwC
test tasks.

Author: jozala

build-tools/src/main/java/org/elasticsearch/gradle/Version.java

@@ -79,10 +79,6 @@ public static Version fromString(final String s, final Mode mode) {
         String revision = matcher.group(3);
         String qualifier = matcher.group(4);
 
-        if (major.equals("0") && minor.equals("0") && revision.equals("0")) {
-            throw new IllegalArgumentException("Version 0.0.0 is not allowed");
-        }
-
         return new Version(Integer.parseInt(major), Integer.parseInt(minor), revision == null ? 0 : Integer.parseInt(revision), qualifier);
     }
 

build-tools/src/test/groovy/org/elasticsearch/gradle/VersionSpec.groovy

@@ -76,12 +76,6 @@ class VersionSpec extends Specification {
         then:
         e = thrown(IllegalArgumentException)
         assert e.message == "Invalid version format: 'foo.bar.baz'. Should be major.minor.revision[-(alpha|beta|rc)Number|-SNAPSHOT]"
-
-        when:
-        Version.fromString("0.0.0")
-        then:
-        e = thrown(IllegalArgumentException)
-        assert e.message == "Version 0.0.0 is not allowed"
     }
 
     def "handles qualifier"() {