The bug in #329 occurred because of a bug in matchSources, where the
filepath wasn't absolute, so the source "/kaniko-bug/*" wasn't being
matched to the file "kaniko-bug/test-file"
To fix this, I added logic for making filepaths absolute and added to
the unit test for the function to test that it works.
When this test was originally created, an HTTP get to
`https://url.com/something/not/real` probably failed, but now it
will return a `503`, i.e. the `http.Get` call will succeed.
This test will now use a URL which should not reasonable ever
succeed (famous last words). Alternatively we could use dependency
injection and mock `http.Get` but it doesn't seem worth it.
This commit also updates the test to use `Run` to run each test
in the table test as a separate test so we can get a clear indication
which cases fail and which succeed.