Delete GZIP environment variable before running interop tests

[pmqs]

Gzip allows default options to be set with the GZIP environment variable.

get the test harness to delete this env variable in CompTestUtils.pm to prevent unexpected results.

Same goes for BZIP, BZIP2 for Bzip2
ZSTD_CLEVEL for zstd.
XZ_DEFAULTS for xz

[pmqs]

Also ZIP, ZIPOPT and TZ for zip
UNZIP and TZ for unzip