make check: test_clear fails

[grubles]

Testing release_0.8.9 on Fedora 39 Branched with gcc 13.0.1.

============================================
   libwallycore 0.8.9: src/test-suite.log
============================================

# TOTAL: 6
# PASS:  5
# SKIP:  0
# XFAIL: 0
# FAIL:  1
# XPASS: 0
# ERROR: 0

.. contents:: :depth: 2

FAIL: test_clear
================

Found bip39_mnemonic_validate secret at stack position 11024 and base 0x7e52a0
7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 7f, 
test_bip39 wally_clear() test failed!
FAIL test_clear (exit status: 1)

[jgriffiths]

@grubles this is a known problem with some compilers which see through our test shenanigans and optimise out the calls. per README.md configure args:

- `--disable-clear-tests`. Disables just the test_clear test (required to pass
  the test suite with some compilers).