Verification of TerminalInterface::terminalProcessId method functionality in TerminalInterfaceTest

This change attempts to resolve a "FIXME: find a way to verify this". Currently, tests to check if TerminalInterface::terminalProcessId returns a id of an appropriate process are yet to be implemented. For Linux platforms, this change resolves this issue by verifying that TerminalInterface::terminalProcessId returns an id for a process with an executable that has a path matching the user default shell set in /etc/passwd.