Expand test coverage for error-handling macros and error pool

Add 11 CTest programs and a shared test helper covering gaps left by the
original four tests (which only exercised FAIL/CATCH/HANDLE and CLEANUP):

- err_capture.h: capturing akerr_log_method + NDEBUG-proof AKERR_CHECK so
  tests can assert on message/status/stacktrace content, not just exit codes
- err_success: clean nested return does not break/handle/leak
- err_pool_refcount: 100k raise->catch->handle cycles leak 0 pool slots
- err_handle_default / err_handle_group / err_handle_dispatch: handler routing
- err_pass / err_ignore / err_swallow: PASS, IGNORE, FINISH(e,false)
- err_break_variants: FAIL_*_BREAK and FAIL_*_RETURN
- err_errno: system errno name lookup + "Unknown Error" boundary
- err_custom_handler: override the unhandled-error hook, assert non-fatally

Register tests via a foreach loop in CMakeLists.txt. Ignore build/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/err_capture.h

@@ -0,0 +1,84 @@
+#ifndef AKERR_TEST_CAPTURE_H
+#define AKERR_TEST_CAPTURE_H
+
+/*
+ * Shared test helpers for libakerror.
+ *
+ * Installs a capturing implementation of akerr_log_method so tests can assert
+ * on the *content* of log/stacktrace output (messages, status codes, error
+ * names) instead of relying solely on process exit codes.
+ *
+ * Also provides AKERR_CHECK(), a NDEBUG-proof assertion that fails the test by
+ * returning non-zero from main() (unlike assert(), which is compiled out in
+ * release builds and would silently turn a test into a no-op).
+ */
+
+#include "akerror.h"
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+
+#define AKERR_CAPTURE_BUFSZ 65536
+static char akerr_capture_buf[AKERR_CAPTURE_BUFSZ];
+static size_t akerr_capture_len = 0;
+
+static void __attribute__((unused)) akerr_capture_logger(const char *fmt, ...)
+{
+    va_list ap;
+    va_start(ap, fmt);
+    int n = vsnprintf(akerr_capture_buf + akerr_capture_len,
+		      AKERR_CAPTURE_BUFSZ - akerr_capture_len, fmt, ap);
+    va_end(ap);
+    if ( n > 0 ) {
+	akerr_capture_len += (size_t)n;
+	if ( akerr_capture_len >= AKERR_CAPTURE_BUFSZ ) {
+	    akerr_capture_len = AKERR_CAPTURE_BUFSZ - 1;
+	}
+    }
+}
+
+static void __attribute__((unused)) akerr_capture_reset(void)
+{
+    akerr_capture_len = 0;
+    akerr_capture_buf[0] = '\0';
+}
+
+/*
+ * Install the capturing logger. akerr_init() only assigns a default logger when
+ * akerr_log_method is NULL, and it is idempotent, so calling this either before
+ * or after the first PREPARE_ERROR keeps our logger in place.
+ */
+static void __attribute__((unused)) akerr_capture_install(void)
+{
+    akerr_capture_reset();
+    akerr_log_method = &akerr_capture_logger;
+}
+
+/* Count array slots currently checked out of the pool (refcount != 0). */
+static int __attribute__((unused)) akerr_slots_in_use(void)
+{
+    int n = 0;
+    for ( int i = 0; i < AKERR_MAX_ARRAY_ERROR; i++ ) {
+	if ( AKERR_ARRAY_ERROR[i].refcount != 0 ) {
+	    n++;
+	}
+    }
+    return n;
+}
+
+#define AKERR_CHECK(cond)						\
+    do {								\
+	if ( !(cond) ) {						\
+	    fprintf(stderr, "CHECK FAILED: %s at %s:%d\n",		\
+		    #cond, __FILE__, __LINE__);				\
+	    return 1;							\
+	}								\
+    } while ( 0 )
+
+#define AKERR_CHECK_CONTAINS(needle)					\
+    AKERR_CHECK(strstr(akerr_capture_buf, (needle)) != NULL)
+
+#define AKERR_CHECK_NOT_CONTAINS(needle)				\
+    AKERR_CHECK(strstr(akerr_capture_buf, (needle)) == NULL)
+
+#endif // AKERR_TEST_CAPTURE_H