[lldb] Add expect_var_path to test variable path results

This adds `expect_var_path` to test variable paths so we no longer have to
use `frame var` and find substrs in the command output. The behaviour
is identical with `expect_expr` (and it also uses the same checking backend),
but it instead calls `GetValueForVariablePath` to evaluate the string as a variable
path.

Also rewrites a few of the tests that previously used `frame variable` to use
`expect_var_path`.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D90450