Require test_rule arg (#46)

Few callers, might as well

buildifier/factory.bzl

diff --git a/buildifier/factory.bzl b/buildifier/factory.bzl
index 1d9b1e4..ccd0a29 100644
--- a/buildifier/factory.bzl
+++ b/buildifier/factory.bzl
@@ -4,7 +4,7 @@
 
 load("@bazel_skylib//lib:shell.bzl", "shell")
 
-def buildifier_attr_factory(test_rule = False):
+def buildifier_attr_factory(*, test_rule):
     """
     Helper macro to generate a struct of attrs for use in a rule() definition.
 
@@ -78,7 +78,7 @@
 
     return attrs
 
-def buildifier_impl_factory(ctx, test_rule = False):
+def buildifier_impl_factory(ctx, *, test_rule):
     """
     Helper macro to generate a buildifier or buildifier_test rule.
 

rules.bzl

diff --git a/rules.bzl b/rules.bzl
index a9fb9b7..a69d7b8 100644
--- a/rules.bzl
+++ b/rules.bzl
@@ -14,11 +14,11 @@
 buildozer_binary = _buildozer_binary
 
 def _buildifier_impl(ctx):
-    return [buildifier_impl_factory(ctx)]
+    return [buildifier_impl_factory(ctx, test_rule = False)]
 
 buildifier = rule(
     implementation = _buildifier_impl,
-    attrs = buildifier_attr_factory(),
+    attrs = buildifier_attr_factory(test_rule = False),
     toolchains = ["@buildifier_prebuilt//buildifier:toolchain"],
     executable = True,
 )
@@ -28,7 +28,7 @@
 
 buildifier_test = rule(
     implementation = _buildifier_test_impl,
-    attrs = buildifier_attr_factory(True),
+    attrs = buildifier_attr_factory(test_rule = True),
     toolchains = ["@buildifier_prebuilt//buildifier:toolchain"],
     test = True,
 )