scalafix-testkit: expose --save-expect helper

[bjaglin]

When adding or maintaining integration tests via scalafix-testkit, it can be useful to dump the "actual" result of running the rules to the "expected" files, in the output directory.

This is what

scalafix/scalafix-tests/unit/src/test/scala/scalafix/tests/rule/RuleSuite.scala

Lines 8 to 29 in 17a187c

	def main(args: Array[String]): Unit = {
	if (Array("--save-expect").sameElements(args)) {
	val suite = new SemanticRuleSuite(
	TestkitProperties.loadFromResources(),
	isSaveExpect = true
	) {
	testsToRun.foreach { t =>
	try evaluateTestBody(t)
	catch {
	case _: TestFailedException =>
	case NonFatal(e) =>
	e.printStackTrace()
	}
	}
	}
	println("Promoted expect tests")
	} else {
	println(
	s"unknown arguments '${args.mkString(" ")}', expected '--save-expect'"
	)
	}
	}

does. This could be made more accessible and documented by exposing a dedicated task/command in sbt-scalafix ScalafixTestkitPlugin.

Triggered by https://github.com/scalacenter/sbt-scalafix/pull/120/files#r434473409.