Replace file.write and os.path.exists by mock

During the test files may be lost in the temporary directory.
Therefore, it would be nice if we reduce dependence of temporary directory.

This patch replace buildin method file.write by mock in ShellCacheSchemaTest.
Also we need to replace method os.path.exists by mock to correct all checks.

Fixes bug: #1267515

Change-Id: I3faca27348e15b28619e49f11d0d6cff5a9e2906
Merge "Replace file.write and os.path.exists by mock"