tests: use $(rootpaths) to get executable files paths for Bazel 8 compatibility (#2395)

In Bazel 8, the singular `$(rootpath)` expansions require that the
target expands to a
single file. The py rules have an unfortunate legacy behavior where
their default outputs
are the executable and the main py file, thus causing an error.

To fix, use the plural `$(rootpaths)`, then post-process the
space-separated string to get
just the executable portion of it.

Along the way...
* Add tests/integration/py_cc_toolchain_registered/bazel-* symlink to
Bazel ignore.
This avoids an infinite symlink expansions error and performance issues
when those
  symlinks exist.

Work towards #2378

---------

Co-authored-by: Ivo List <[email protected]>

Author: rickeylev

.bazelignore

@@ -26,3 +26,4 @@ examples/py_proto_library/bazel-py_proto_library
 tests/integration/compile_pip_requirements/bazel-compile_pip_requirements
 tests/integration/ignore_root_user_error/bazel-ignore_root_user_error
 tests/integration/local_toolchains/bazel-local_toolchains
+tests/integration/py_cc_toolchain_registered/bazel-py_cc_toolchain_registered

examples/bzlmod/MODULE.bazel.lock

@@ -163,7 +163,7 @@ sh_test(
     data = [":version_default"],
     env = {
         "VERSION_CHECK": "3.9",  # The default defined in the WORKSPACE.
-        "VERSION_PY_BINARY": "$(rootpath :version_default)",
+        "VERSION_PY_BINARY": "$(rootpaths :version_default)",
     },
 )
 
@@ -173,7 +173,7 @@ sh_test(
     data = [":version_3_9"],
     env = {
         "VERSION_CHECK": "3.9",
-        "VERSION_PY_BINARY": "$(rootpath :version_3_9)",
+        "VERSION_PY_BINARY": "$(rootpaths :version_3_9)",
     },
 )
 
@@ -183,6 +183,6 @@ sh_test(
     data = [":version_3_10"],
     env = {
         "VERSION_CHECK": "3.10",
-        "VERSION_PY_BINARY": "$(rootpath :version_3_10)",
+        "VERSION_PY_BINARY": "$(rootpaths :version_3_10)",
     },
 )

examples/bzlmod/tests/BUILD.bazel

@@ -16,7 +16,11 @@
 
 set -o errexit -o nounset -o pipefail
 
-version_py_binary=$("${VERSION_PY_BINARY}")
+# VERSION_PY_BINARY is a space separate list of the executable and its main
+# py file. We just want the executable.
+bin=($VERSION_PY_BINARY)
+bin="${bin[@]//*.py}"
+version_py_binary=$($bin)
 
 if [[ "${version_py_binary}" != "${VERSION_CHECK}" ]]; then
     echo >&2 "expected version '${VERSION_CHECK}' is different than returned '${version_py_binary}'"