Enable forking when running tests.

The Scalafix test suite uses `this.getClass.getClassLoader` to lookup
symbols from scala-library.jar. In sbt v1.3.x, the default classloader
layering strategy changed so that `this.getClass.getClassLoader` only
contains Scalafix-defined classes, with classes from dependency classes
coming from a parent classloader.

This commit makes the tests pass on sbt v1.3.x by enabling forking so
that `this.getClass.getClassLoader` is always a flat classloader
containing all classes.

build.sbt

@@ -4,7 +4,7 @@ inThisBuild(
     onLoadMessage := s"Welcome to scalafix ${version.value}",
     scalaVersion := scala212,
     crossScalaVersions := List(scala212, scala211),
-    fork.in(Test, test) := true
+    fork := true
   )
 )
 
@@ -169,7 +169,8 @@ lazy val unit = project
   .in(file("scalafix-tests/unit"))
   .settings(
     noPublish,
-    fork := false,
+    // Change working directory to match when `fork := false`.
+    baseDirectory.in(Test) := baseDirectory.in(ThisBuild).value,
     javaOptions := Nil,
     buildInfoPackage := "scalafix.tests",
     buildInfoObject := "BuildInfo",
@@ -251,6 +252,7 @@ lazy val docs = project
   .in(file("scalafix-docs"))
   .settings(
     noMima,
+    baseDirectory.in(run) := baseDirectory.in(ThisBuild).value,
     skip in publish := true,
     moduleName := "scalafix-docs",
     scalaVersion := scala212,

scalafix-tests/unit/src/test/scala-2.12/scalafix/tests/cli/ScalafixImplSuite.scala

@@ -82,7 +82,7 @@ class ScalafixImplSuite extends FunSuite with DiffAssertions {
   }
 
   test("error") {
-    val cl = new URLClassLoader(Array())
+    val cl = new URLClassLoader(Array(), null)
     val ex = intercept[ScalafixException] {
       i.Scalafix.classloadInstance(cl)
     }