test: Location expand environment variables

[hovind]

Description

Perform location expansion on testing environment.

Motivation

I want to provide paths to the scala_test via the environment. https://github.com/ucb-bar/chiseltest requires VERILATOR_ROOT to be present in the environment. I want to use the @verilator//:verilator_executable provided by https://github.com/hdl/bazel_rules_hdl.

I removed some ifs and elses whose necessity I did not quite understand, please advice if I have broken something.

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