test: avoid undersized Boost.Test signal stacks

Boost.Test registers a `SIGSTKSZ` alternate signal stack as each test binary starts, before any tests run.
On musl, this can be smaller than Linux's hardware-dependent minimum, causing `sigaltstack()` to fail with `ENOMEM` during setup.

Disable Boost.Test's alternate stack in both test entry points.
Signal handlers continue to use the regular process stack. Boost.Test can no longer report stack overflows.

Fixes #36026

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>

src/test/kernel/test_kernel.cpp

@@ -7,6 +7,8 @@
 #include <util/byte_units.h>
 #include <util/fs.h>
 
+// Boost.Test's SIGSTKSZ alternate stack can be smaller than Linux requires on musl.
+#define BOOST_TEST_DISABLE_ALT_STACK
 #define BOOST_TEST_MODULE Bitcoin Kernel Test Suite
 #include <boost/test/included/unit_test.hpp>
 

src/test/main.cpp

@@ -5,6 +5,8 @@
 /**
  * See https://www.boost.org/doc/libs/1_78_0/libs/test/doc/html/boost_test/adv_scenarios/single_header_customizations/multiple_translation_units.html
  */
+// Boost.Test's SIGSTKSZ alternate stack can be smaller than Linux requires on musl.
+#define BOOST_TEST_DISABLE_ALT_STACK
 #define BOOST_TEST_MODULE Bitcoin Core Test Suite
 
 #include <boost/test/included/unit_test.hpp>