[lldb][test] Replace use of p with expression in Shell tests (NFC)

In Shell tests, replace use of the `p` alias with the `expression` command.

To avoid conflating tests of the alias with tests of the expression command,
this patch canonicalizes to the use `expression`.

See also D141539 which made the same change to API tests.

Differential Revision: https://reviews.llvm.org/D146230