Fix yet another gradle deprecation, expand tests to cover it

src/main/java/net/fabricmc/loom/task/UnpickJarTask.java

@@ -47,7 +47,7 @@ public class UnpickJarTask extends JavaExec {
 	public UnpickJarTask() {
 		getOutputs().upToDateWhen(e -> false);
 		classpath(getProject().getConfigurations().getByName(Constants.Configurations.UNPICK_CLASSPATH));
-		setMain("daomephsta.unpick.cli.Main");
+		getMainClass().set("daomephsta.unpick.cli.Main");
 	}
 
 	@Override

src/test/groovy/net/fabricmc/loom/test/integration/UnpickTest.groovy

@@ -42,17 +42,25 @@ class UnpickTest extends Specification implements ProjectTestTrait {
 
 	def "unpick decompile"() {
 		when:
-			def result = create("genSources")
+			def result = create("genSources", gradle)
 		then:
 			result.task(":genSources").outcome == SUCCESS
 			getClassSource("net/minecraft/block/CakeBlock.java").contains("Block.DEFAULT_SET_BLOCK_STATE_FLAG")
+		where:
+			gradle              | _
+			DEFAULT_GRADLE      | _
+			PRE_RELEASE_GRADLE  | _
 	}
 
 	def "unpick build"() {
 		when:
-			def result = create("build")
+			def result = create("build", gradle)
 		then:
 			result.task(":build").outcome == SUCCESS
+		where:
+			gradle              | _
+			DEFAULT_GRADLE      | _
+			PRE_RELEASE_GRADLE  | _
 	}
 
 	String getClassSource(String classname, String mappings = MAPPINGS) {

src/test/groovy/net/fabricmc/loom/test/util/ProjectTestTrait.groovy

@@ -29,7 +29,7 @@ import org.gradle.testkit.runner.GradleRunner
 
 trait ProjectTestTrait {
 	final static String DEFAULT_GRADLE = "7.0.1"
-	final static String PRE_RELEASE_GRADLE = "7.2-20210703221006+0000"
+	final static String PRE_RELEASE_GRADLE = "7.2-20210704221017+0000"
 
 	static File gradleHome = File.createTempDir()
 	File testProjectDir = File.createTempDir()