So far it was impossible to put absolute paths into files, as one
wouldn't know whre the test would be placed.
This commit adds a new Fileish variant called `EnvTemplate` which uses
the given string as a template and replaces the `@ENV_VARIABLE_NAME@`
placeholders with the according value of the environment variable.
This way one can use `@HOME@` or `@TEST_ROOT@` to build absolute paths
Change-Id: I425cb6408dceb8d7f26d136ace4ac98b1ca31ec3
Commentator2.0
acf6e1c6b7