[lldb] Simplify and improve FileSpecTest

Summary:
A most of these tests create FileSpecs with a hardcoded style. Add
utility functions which create a file spec of a given style to simplify
things.

While in there add SCOPED_TRACE messages to tests which loop over
multiple inputs to ensure it's clear which of the inputs failed.

Reviewers: teemperor

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70814