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-error.zeek

@@ -0,0 +1,27 @@
+# @TEST-DOC: Assert statement wrong usage
+#
+# @TEST-EXEC-FAIL: zeek -b %INPUT
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+
+event zeek_init()
+	{
+	assert 1;
+	}
+
+@TEST-START-NEXT
+event zeek_init()
+	{
+	assert T, 1234;
+	}
+
+@TEST-START-NEXT
+event zeek_init()
+	{
+	assert;
+	}
+
+@TEST-START-NEXT
+event zeek_init()
+	{
+	assert T, "extra", "something";
+	}