Fix tests on systems where platlib and purelib are different (#13727)

The following tests expect installed files to be in purelib:

* `tests/functional/test_check.py::test_check_unsupported`
* `tests/functional/test_install_wheel.py::test_basic_install_from_wheel`

However, the actual wheels they install have `Root-Is-Purelib` set to
`false` (or missing entirely), which causes the contents to be installed to
platlib instead. Since the wheels in question don't have any
platform-dependent files, it seems appropriate to update them to use
`Root-Is-Purelib: true`.

Since we're touching `has.script-1.0-py2.py3-none-any.whl`, fix the `Tag`
field as well, which was set incorrectly (to `cp26-none-linux_x86_64`).

Author: SpecLad

news/72b8078a-4881-4da4-8b3d-c9bab16eb572.trivial.rst

@@ -1152,7 +1152,15 @@ def add_file(path: str, text: str) -> None:
     records = [(record_path, "", "")]
     buf = BytesIO()
     with ZipFile(buf, "w") as z:
-        add_file(f"{dist_info}/WHEEL", "Wheel-Version: 1.0")
+        add_file(
+            f"{dist_info}/WHEEL",
+            dedent(
+                """\
+                Wheel-Version: 1.0
+                Root-Is-Purelib: true
+                """
+            ),
+        )
         add_file(
             f"{dist_info}/METADATA",
             dedent(