test(venv): Move test_link.py into the py_venv/tests (#1307)

This relocates the `test_link` target and its source file from
`py/private/py_venv/ into py/private/py_venv/tests/`, keeping the test
code alongside the rest of the package's tests. The `py_test` source
paths and imports are updated accordingly.

---

### Changes are visible to end-users: no


### Test plan

- Covered by existing test cases

Author: xangcastle

py/private/py_venv/BUILD.bazel

@@ -1,6 +1,5 @@
 load("@bazel_lib//:bzl_library.bzl", "bzl_library")
 load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
-load("//py:defs.bzl", "py_test")
 
 package(default_visibility = ["//py:__subpackages__"])
 
@@ -90,13 +89,3 @@ bzl_library(
         ":types",
     ],
 )
-
-py_test(
-    name = "test_link",
-    srcs = [
-        "link.py",
-        "test_link.py",
-    ],
-    imports = ["."],
-    main = "test_link.py",
-)

py/private/py_venv/tests/BUILD.bazel

@@ -1,8 +1,16 @@
 load("@bazel_lib//:bzl_library.bzl", "bzl_library")
 load("@bazel_lib//lib:write_source_files.bzl", "write_source_files")
+load("//py:defs.bzl", "py_test")
 load("//py/private/py_venv:defs.bzl", "py_venv", "py_venv_exec")
 load(":venv_tree.bzl", "venv_tree")
 
+bzl_library(
+    name = "venv_tree",
+    srcs = ["venv_tree.bzl"],
+    visibility = ["//py/private/py_venv:__subpackages__"],
+    deps = ["//py/private/py_venv:defs"],
+)
+
 py_venv(
     name = "venv",
     testonly = True,
@@ -45,9 +53,12 @@ write_source_files(
     },
 )
 
-bzl_library(
-    name = "venv_tree",
-    srcs = ["venv_tree.bzl"],
-    visibility = ["//py/private/py_venv:__subpackages__"],
-    deps = ["//py/private/py_venv:defs"],
+py_test(
+    name = "test_link",
+    srcs = [
+        "//py/private/py_venv:link.py",
+        "//py/private/py_venv/tests:test_link.py",
+    ],
+    imports = [".."],
+    main = "//py/private/py_venv/tests:test_link.py",
 )