Ensure MockRoot test folder is always cleared before tests run

Several of Scala Steward's tests (eg `PullRequestRepositoryTest`) write
files to the filesystem, using `MockConfig.mockRoot` as the base folder.

Previously, the folder was erased by using sbt configuration:
`Tests.Cleanup` in the `testOptions` setting (see
https://www.scala-sbt.org/1.x/docs/Testing.html#Setup+and+Cleanup ),
but as documented in https://youtrack.jetbrains.com/issue/SCL-21353
this doesn't work well with IntelliJ's quickly-run-individual-test
feature.

This change ensures that running the tests with IntelliJ, or with sbt,
both work: we now delete the `mockRoot` folder in Scala, rather than
sbt configuration. We also do it _before_ tests run, rather than after,
which I think is better - sometimes you want to examine the files after
tests have failed, and it's only when you're about to run the tests that
you want the previous files deleted.

See also:

#3101 (comment)

Author: rtyley

build.sbt

@@ -212,8 +212,6 @@ lazy val core = myCrossProject("core")
     // Uncomment for remote debugging:
     // run / javaOptions += "-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=5005",
     Test / fork := true,
-    Test / testOptions +=
-      Tests.Cleanup(() => Path(file(Properties.tmpDir) / "scala-steward").deleteRecursively()),
     Compile / resourceGenerators += Def.task {
       val outDir = (Compile / resourceManaged).value
       def downloadPlugin(v: String): File = {

modules/core/src/test/scala/org/scalasteward/core/mock/MockConfig.scala

@@ -6,6 +6,7 @@ import org.scalasteward.core.application.{Cli, Config}
 
 object MockConfig {
   val mockRoot: File = File.temp / "scala-steward"
+  mockRoot.delete(true) // Ensure folder is cleared of previous test files
   private val args: List[String] = List(
     s"--workspace=$mockRoot/workspace",
     s"--repos-file=$mockRoot/repos.md",