Separate Java Toolchain for tests

to make sure that all the versions we release are compatible with all
Java versions we are claiming to support, set a separate Java Toolchain
for tests executions.

* Expose `javaVersion` as project property to be able to control such a
Java version externally, e.g., via GitHub Actions

**Auto-cherry-pick to `4.0.x` & `3.2.x`**

Author: artembilan

build.gradle

@@ -222,6 +222,10 @@ configure(javaProjects) { subproject ->
 
 		useJUnitPlatform()
 
+		javaLauncher = javaToolchains.launcherFor {
+			languageVersion = JavaLanguageVersion.of(project.hasProperty('javaVersion') ? project.javaVersion as int : 25)
+		}
+
 		jvmArgs '--enable-native-access=ALL-UNNAMED'
 	}