Sandbox tests

[rosik]

We lack the way to run tests in their personal sandboxes. Nowadays the flow sequence is approximately the following:

require('test1')
reqture('test2')

test1.before_all()
test1.test()
test1.after_all()

test2.before_all()
test2.test()
test2.after_all()

If the first test modifies global environment (applies mocks) it may affect subsequent tests in a rather puzzled way. See also https://github.com/tarantool/cartridge/pull/1285