test: helper to skip tests depending on 'gzip'

Some of our tests explicitly depends on the 'gzip' command line utility,
for example the Dumps test suite.  This patch adds up a simple helper
MediaWikiTestCase::checkHasGzip() which skips tests whenever gzip is
missing.

Change-Id: I5ded9a53d08dcf5f35e666f91e3f64e6e111af32
Merge "test: helper to skip tests depending on 'gzip'"