test signed left-shift overflow for C99

[kroening]

The semantics of signed left shifts are contentious for the case
that a '1' is shifted into the signed bit. This tests the C99 case.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: ba34966).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96508556