allow to skip online tests

[remicollet]

Sometime we need to be able to run test suite from an offline computer (which, at least, is the case in Fedora build system).

The SKIP_ONLINE_TESTS env. variable is quite common (e.g. used in php test suite)

[JanTvrdik]

Why not just

if (!getenv('SKIP_ONLINE_TESTS')) {
    FileSystem::copy('http://example.com', TEMP_DIR . '/3/x/y/file');
    Assert::true(is_file(TEMP_DIR . '/3/x/y/file'));
}

?

[remicollet]

else is needed because '/3/x/y/file' is expected to exists in file "copy" test.

[milo]

Alternative implementation #93

[dg]

Should be closed by #93