Shell test changes. Use @shell.command instead directly calling functions in admin.rb and other libraries.

Testing by executing a command will cover the exact path users will trigger, so its better then directly calling library functions in tests. Changing the tests to use @shell.command(:<command>, args) to execute them like it's a command coming from shell.

Norm change:
Commands should print the output user would like to see, but in the end, should also return the relevant value. This way:

• Tests can use returned value to check that functionality works
• Tests can capture stdout to assert particular kind of output user should see.
• We do not print the return value in interactive mode and keep the output clean. See Shell.command() function.

Bugs found due to this change:

• Uncovered bug in major_compact.rb with this approach. It was calling admin.majorCompact() which doesn't exist but our tests didn't catch it since they directly tested admin.major_compact()
• Enabled TestReplicationShell. If it's bad, flaky infra will take care of it.