* Modules: Deduplicate suffixes but don't sort them.
The suffixes' order is defined by the order in which they appear in the configuration file.
* Modules: Modify tests to use spack_yaml.load_config.
spack_yaml.load_config ensures that the configuration is stored in an ordered manner. Without this change, the behavior of the tests did not match Spack's.
* Modules: Tweak the suffixes test to better catch ordering issues.
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