bazel: Mark junit targets testonly

Only testonly targets (such as tests) need to use junit.

In particular this involves making the toplevel :all rule testonly.
It's not clear to me what that rule is for --- "bazel build //..."
already works to build all targets.  In any case it appears to be for
testing, so marking it as testonly shouldn't be harmful.

Change-Id: I28ff508ab8ce2ec0a0111109110aa9680d30600e

BUILD

@@ -2,6 +2,7 @@ package(default_visibility = ["//visibility:public"])
 
 genrule(
     name = "all",
+    testonly = 1,
     srcs = [
         "//org.eclipse.jgit:jgit",
         "//org.eclipse.jgit.archive:jgit-archive",

lib/BUILD

@@ -39,7 +39,8 @@ java_library(
 
 java_library(
     name = "junit",
-    visibility = ["//org.eclipse.jgit.junit:__pkg__"],
+    testonly = 1,
+    visibility = ["//visibility:public"],
     exports = ["@junit//jar"],
 )
 

org.eclipse.jgit.junit/BUILD

@@ -2,6 +2,7 @@ package(default_visibility = ["//visibility:public"])
 
 java_library(
     name = "junit",
+    testonly = 1,
     srcs = glob(["src/**"]),
     resource_strip_prefix = "org.eclipse.jgit.junit/resources",
     resources = glob(["resources/**"]),