Those long indentifiers make test output very wide, and they are ultimately
not very useful for humans to look at. Let's use some short string to identify
the test failure instead.
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=33881.
Not only we would duplicate unknown input on the stack, we would do it
over and over. So let's first check that the input has reasonable length,
but also allocate just one fixed size buffer.
