proto/test: relax stddev check in test_script_typed_ir_size

Context

The test test_script_typed_ir_size is flaky despite fixing a seed. This is because some underlying code uses Random.stuff instead of Random.State.stuff. The easy fix is to relax the check and hence decrease flakyness to non-perceptible levels.

Observe the flakyness here: https://gitlab.com/nomadic-labs/tezos/-/jobs/1604002195

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
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