[2.x] test: Add a regression test for incremental test (#9608)

**Problem/Solution**
This tests incremental test's stability post-version bump.

sbt-app/src/sbt-test/tests/version-bump-test/app/src/test/scala/FooTest.scala

@@ -0,0 +1,9 @@
+package example
+
+import java.nio.file.{ Files, Paths }
+
+class FooTest extends munit.FunSuite:
+  test("greeting"):
+    Files.write(Paths.get("test-ran.marker"), "ran".getBytes)
+    assertEquals(Foo.greeting, "hello")
+end FooTest

sbt-app/src/sbt-test/tests/version-bump-test/build.sbt

@@ -0,0 +1,16 @@
+Global / localCacheDirectory := baseDirectory.value / "diskcache"
+
+scalaVersion := "3.8.4"
+
+val munit = "org.scalameta" %% "munit" % "1.0.4"
+
+lazy val foo = project
+
+lazy val app = project
+  .dependsOn(foo)
+  .settings(
+    libraryDependencies += munit % Test
+  )
+
+lazy val root = (project in file("."))
+  .aggregate(foo, app)

sbt-app/src/sbt-test/tests/version-bump-test/foo/Foo.scala

@@ -0,0 +1,5 @@
+package example
+
+object Foo:
+  def greeting: String = "hello"
+end Foo

sbt-app/src/sbt-test/tests/version-bump-test/test

@@ -0,0 +1,20 @@
+# the test suite writes this file as a side effect whenever it actually runs
+$ absent test-ran.marker
+
+# warm the disk cache
+> app/test
+$ exists test-ran.marker
+$ delete test-ran.marker
+
+# after a clean, retesting app should fully restore from the disk cache;
+# since nothing changed, the test body must not execute again
+> clean
+> app/test
+$ absent test-ran.marker
+
+# bumping foo's version alone (e.g. what sbt-dynver would do on a new commit) must not
+# invalidate app's test cache entry, since foo's jar content hasn't changed
+> set foo / version := "0.2.0"
+> clean
+> app/test
+$ absent test-ran.marker