tests: add coverage for new --zstd and all other compression tools

tests: add coverage for new --zstd and all other compression tools

* tests/compress.at.in: New file: template from which each
per-compression-tool test is derived.
* tests/Makefile.am (TESTSUITE_AT): Add it.
(EXTRA_DIST): Add compress.at.in.
(compress-*.at): New rules to generate a file/tests for each compression tool.
* tests/testsuite.at (Compression): Add each of these generated
files in a new section.
* tests/.gitignore: Ignore the new generated files.