Use TESTS_TARGET_DIR envvar to override tests target dir

[silwol]

Rationale: At Debian, the autopkgtests are run inside a non-writable directory, so the "target" subdirectory can't be used. Inside the autopkgtest environment, we have an env variable AUTOPKGTEST_TMP available which points to a temporary directory, that we can pass on to the tests as TESTS_TARGET_DIR. This makes the tests pass properly inside this environment.

[cuviper]

Looks good, thanks!

bors r+

[bors[bot]]

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