Update {junit,specs2_junit}_toolchain()

These macros now mirror the implementation of `scalatest_toolchain()`.

However, I realized that the old pattern of calling
`scalatest_repositories()` followed by `scalatest_toolchain()` will no
longer work without first calling `scala_toolchains(scalatest = True)`.
This is because the `alias` targets in `testing/BUILD` that replace the
previous implementations all point to
`@io_bazel_rules_scala_toolchains`.

So if we want to keep these macros, it seems like we should maybe
restore the original toolchain targets in `testing/BUILD`. If we don't,
we can remove these macros, but we can document these as breaking
changes, and update other documentation accordingly.

testing/junit.bzl

@@ -1,7 +1,12 @@
 load("//junit:junit.bzl", _repositories = "junit_repositories")
+load("//scala:scala_cross_version.bzl", "version_suffix")
+load("@io_bazel_rules_scala_config//:config.bzl", "SCALA_VERSIONS")
 
 def junit_repositories():
     _repositories()
 
 def junit_toolchain():
-    native.register_toolchains("@io_bazel_rules_scala//testing:junit_toolchain")
+    for scala_version in SCALA_VERSIONS:
+        native.register_toolchains(str(Label(
+            "//testing:junit_toolchain" + version_suffix(scala_version),
+        )))

testing/scalatest.bzl

@@ -1,5 +1,5 @@
-load("//scalatest:scalatest.bzl", _repositories = "scalatest_repositories")
 load("//scala:scala_cross_version.bzl", "version_suffix")
+load("//scalatest:scalatest.bzl", _repositories = "scalatest_repositories")
 load("@io_bazel_rules_scala_config//:config.bzl", "SCALA_VERSIONS")
 
 def scalatest_repositories():
@@ -8,6 +8,5 @@ def scalatest_repositories():
 def scalatest_toolchain():
     for scala_version in SCALA_VERSIONS:
         native.register_toolchains(str(Label(
-            "//testing:scalatest_toolchain" +
-            version_suffix(scala_version),
+            "//testing:scalatest_toolchain" + version_suffix(scala_version),
         )))

testing/specs2_junit.bzl

@@ -1,7 +1,12 @@
+load("//scala:scala_cross_version.bzl", "version_suffix")
 load("//specs2:specs2_junit.bzl", _repositories = "specs2_junit_repositories")
+load("@io_bazel_rules_scala_config//:config.bzl", "SCALA_VERSIONS")
 
 def specs2_junit_repositories():
     _repositories()
 
 def specs2_junit_toolchain():
-    native.register_toolchains("@io_bazel_rules_scala//testing:specs2_junit_toolchain")
+    for scala_version in SCALA_VERSIONS:
+        native.register_toolchains(str(Label(
+            "//testing:specs2_junit_toolchain" + version_suffix(scala_version),
+        )))