test: copy out the necessary test data before we start overmounting stuff

Otherwise the get_testdata_dir() call fails if the source tree is under
/root (which is usually the case in CIs).

I got bitten by this after leaving the source tree under /root but moving the
$BUILD_DIR elsewhere. This used to work by accident, as load_testdata_env()
would try to read $BUILD_DIR/systemd-runtest.env, but would fail if the
$BUILD_DIR is also under /root and fall back to SYSTEMD_TEST_DATA
(/lib/systemd/tests/testdata), which usually exist as we install the just built
revision. However, if the $BUILD_DIR is outside of /root we'd read
$BUILD_DIR/systemd-runtest.env which contains
SYSTEMD_TEST_DATA=/path/to/source/tree/test and that source tree is not visible
once we overmount /root with tmpfs making the test fail:

/* test_run_tests_unprivileged */
Successfully forked off '(test-execute-unprivileged)' as PID 10672.
Changing mount flags / (MS_REMOUNT|MS_BIND "")...
Changing mount propagation / (MS_REC|MS_SHARED "")
Mounting tmpfs (tmpfs) on /dev/shm (MS_NOSUID|MS_NODEV "")...
Mounting tmpfs (tmpfs) on /root (MS_NOSUID|MS_NODEV "")...
Mounting tmpfs (tmpfs) on /tmp (MS_NOSUID|MS_NODEV "")...
Mounting tmpfs (tmpfs) on /var/tmp (MS_NOSUID|MS_NODEV "")...
Mounting tmpfs (tmpfs) on /var/lib (MS_NOSUID|MS_NODEV "")...
Mounting tmpfs (tmpfs) on /run/test-execute-unit-dir (MS_NOSUID|MS_NODEV "")...
ERROR: $SYSTEMD_TEST_DATA directory [/root/systemd/test] not accessible: No such file or directory
Assertion 'get_testdata_dir("test-execute/", &unit_dir) >= 0' failed at src/test/test-execute.c:1306, function prepare_ns(). Aborting.
(test-execute-unprivileged) terminated by signal ABRT.

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