openpgp: secret_leak_detector tests randomly fail

It appears that the newly added "leak detection" tests are not very reliable. Attempting to build sequoia-openpgp v1.17.0 on Fedora, I got two different failures on two runs:

• first attempt:

clean_basecase: running test
[stack]: 139264 bytes
7ffe5de8de40  00 00 00 00 00 00 00 00  40 40 40 40 40 40 40 40   ........!!!!!!!!
clean_basecase: secret leaked
test leak_tests::clean_basecase ... FAILED

• second attempt

test_encrypted: running test
[stack]: 139264 bytes
7ffee0d59ef0  58 a1 d5 e0 fe 7f 00 00  40 40 40 40 40 40 40 40   ........!!!!!!!!
test_encrypted: secret leaked
test leak_tests::test_encrypted ... FAILED