[abseil] fix test-allocator

[Filyus]

This fixes building librsvg with Cairo on windows for the 20240116.2#2 version, maybe 20240116.2#3 don't needs this already, but probably it needs.

This patch just adds TESTONLY word there:

# Internal-only target, do not depend on directly.
absl_cc_library(
  NAME
    test_allocator
  HDRS
    "internal/test_allocator.h"
  COPTS
    ${ABSL_DEFAULT_COPTS}
  DEPS
    absl::config
    GTest::gmock
  TESTONLY
)

The patch is by SpaceIm and it's taken from there:
https://github.com/conan-io/conan-center-index/blob/832ff6dd9098928c6dd5d222bd780b1240dfed2e/recipes/abseil/all/patches/0004-test-allocator-testonly.patch#L4
with related commit on Conan.io:
conan-io/conan-center-index@a7f4e35

abseil PR - closed and not merged probably because of Contributor License Agreement (CLA):
abseil/abseil-cpp#1536

Related file:
https://github.com/abseil/abseil-cpp/blob/lts_2024_01_16/absl/container/CMakeLists.txt

Package page:
https://vcpkg.io/en/package/abseil

[Filyus]

@microsoft-github-policy-service agree

[dg0yt]

This patch is already included.

[dg0yt]

Guess what 779a356-test-allocator.diff is doing...

[Filyus]

Guess what 779a356-test-allocator.diff is doing...

Sorry, my bad...

[Filyus]

I'll just leave a link to it: #39664

[data-queue]

I think we can close this PR since the patch file is already included

[data-queue]

Closing, see my comment above