Address latent buildifier issues. (#278)

In https://buildkite.com/bazel/bazel-skylib/builds/1240#annotation-buildifier
I noticed buildifier issues unrelated to my PR.
This PR attempts to address these latent buildifier issues.

lib/unittest.bzl

@@ -126,7 +126,10 @@ def _make(impl, attrs = {}):
         toolchains = [TOOLCHAIN_TYPE],
     )
 
-_ActionInfo = provider(fields = ["actions", "bin_path"])
+_ActionInfo = provider(
+    doc = "Information relating to the target under test.",
+    fields = ["actions", "bin_path"],
+)
 
 def _action_retrieving_aspect_impl(target, ctx):
     return [

tests/selects_tests.bzl

@@ -129,7 +129,10 @@ def _set_conditions(condition_list):
         ans["//command_line_option:features"] = ["notmyfeature"]
     return ans
 
-_BooleanInfo = provider()
+_BooleanInfo = provider(
+    doc = "value for boolean tests",
+    fields = ["value"],
+)
 
 def _boolean_attr_impl(ctx):
     return [_BooleanInfo(value = ctx.attr.myboolean)]

tests/unittest_tests.bzl

@@ -54,7 +54,10 @@ def _change_setting_test(ctx):
 
     return analysistest.end(env)
 
-_ChangeSettingInfo = provider()
+_ChangeSettingInfo = provider(
+    doc = "min_os_version for change_setting_test",
+    fields = ["min_os_version"],
+)
 
 def _change_setting_fake_rule(ctx):
     return [_ChangeSettingInfo(min_os_version = ctx.fragments.cpp.minimum_os_version())]
@@ -180,7 +183,10 @@ inspect_actions_test = analysistest.make(
 ########################################
 ####### inspect_output_dirs_test #######
 ########################################
-_OutputDirInfo = provider(fields = ["bin_path"])
+_OutputDirInfo = provider(
+    doc = "bin_path for inspect_output_dirs_test",
+    fields = ["bin_path"],
+)
 
 def _inspect_output_dirs_test(ctx):
     """Test verifying output directories used by a test."""