PamModuleArgValue: add unit test coverage

This commit refactorred the codes such that it becomes possible to unit
test the two non-trivial method: setPamModuleArgValue and
getPamModuleArgValue.

The unit test manipulates temporary files in the unit test container
instead of real pam configs.

Check the coverage data before and after this commit to see how these
two files now get nearly 100% unit test coverage.

Tested: unit test.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Ibad461ace0eae89183de0ddbfb189ee0458b1d2e
3 files changed