[PATCH] tests: Fix xz tests by unsetting XZ_OPT, XZ_DEFAULTS

[Sam James]

Without this, with e.g. XZ_OPT="-T0" in the environment, I get:
--- /dev/null   2023-07-18 10:47:11.976101268 +0100
+++ /home/sam/git/tar/tests/testsuite.dir/at-groups/19/stderr   2023-07-18 
16:45:49.331469505 +0100
@@ -0,0 +1,3 @@
+xz: Reduced the number of threads from 32 to 12 to not exceed the memory usage 
limit of 16055 MiB
+xz: Reduced the number of threads from 32 to 12 to not exceed the memory usage 
limit of 16055 MiB
+xz: Reduced the number of threads from 32 to 12 to not exceed the memory usage 
limit of 16055 MiB
19. testsuite.at:244: 19. compress: xz (testsuite.at:244): FAILED 
(testsuite.at:244)

Signed-off-by: Sam James <sam@gentoo.org>
---
 tests/compress.m4 | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tests/compress.m4 b/tests/compress.m4
index 2866dfac..1e850211 100644
--- a/tests/compress.m4
+++ b/tests/compress.m4
@@ -26,6 +26,8 @@ AT_KEYWORDS([compression compress-$1 $1])
 AT_CHECK([
 AT_GZIP_PREREQ($1)
 unset TAR_OPTIONS
+unset XZ_OPT
+unset XZ_DEFAULTS
 
 touch empty
 set -e
-- 
2.41.0