Stmt: Introduce assert statement and related hooks

including two hooks called assertion_failure() and assertion_result() for
customization and tracking of assertion results.

testing/btest/language/assert-top-level.zeek

@@ -0,0 +1,5 @@
+# @TEST-EXEC-FAIL: zeek -b %INPUT >out
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+
+assert getpid() > 0;
+assert getpid() == 0, fmt("my pid greater 0? %s", getpid() > 0);