Fix generate testing code coverage

[mssun]

Description

Fix inaccuracy of the coverage report. There three main reasons:

• use llvm-cov-11 instead of the default llvm (which may be llvm 10)
• increase heap and stack for the enclave of the integration test
• exclude some unnecessary when capture the coverage info

The new coverage result has been uploaded to coveralls: https://coveralls.io/builds/46626151

Type of change (select or add applied and delete the others)

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

https://ci.mesalock-linux.org/mssun/incubator-teaclave/151

Checklist

• Fork the repo and create your branch from master.
• If you've added code that should be tested, add tests.
• If you've changed APIs, update the documentation.
• Ensure the tests pass (see CI results).
• Make sure your code lints/format.