Add multipass tests

[strarsis]

@GreLI:

• This PR adds tests for multipass in svgo. Currently, there aren't any tests for multipass at all.
• It also fixes a bug with multipassCount that was uncovered with these newly added multipass tests.
• A test for the prefixIds plugin with multipass is added.
• A test for file name handling of the prefixIds plugin is added (it is very similar to the test for prefixIds with multipass, so it seems to be a nice addition).
• A test for the addAttributesToSVGElement plugin with multipass is added.
• Adds a test (albeit retroactively) for invoking svgo.optimize() without passing it an info object.

@GreLI: Additional points:

• During adding the multipass tests I noticed that it doesn't seem to be possible to use a plugins property for the object passed to svgo(), it is simply ignored. enable can't be used for plugins that require extra configuration, like the addAttributesToSVGElement plugin, so I had to pass a raw config string (see https://github.com/svg/svgo/pull/1177/files#diff-58d889243ada560f65a0a1826461d5deR228).
• Also for better isolating the prefixIds plugin related tests, I disabled all other the plugins. But as plugins key seems to be ignored, I had to pass the plugins from svgo defaults to disable (see https://github.com/svg/svgo/pull/1177/files#diff-58d889243ada560f65a0a1826461d5deR177).

Edit: What about using a separate tests folder for the plugin tests with multipass enabled?

[dphil]

With this approach, the first pass has multipassCount as undefined, rather than 0. With the way multipassCount is currently used (the check in prefixIds), it makes no difference, but initializing it to 0 would likely be more appropriate and robust.

[strarsis]

@dphil: multipassCounter is now initialized to 0 at the start: https://github.com/svg/svgo/pull/1177/files#diff-2338951073175db78c5b00c29d4971e1R26

I added the undefined info object test from the other PR to this PR because it fits the whole issue well.

[strarsis]

@GreLI: The build of this PR currently fails for node v6.
This is not caused by this PR but is an actual issue with the current svgo master.
PR for current master to fix failing build on node v6: #1208

[strarsis]

@kylemh: I got an email notification from GitHub with a comment from you about the prefixIds plugin not working correctly with multipass and specific options. I added a test for this set of options and the test passes. Are you using a specific node version? There were node-specific issues in the past.

[kylemh]

I was hasty and incorrect. It had to do with the difference of build-time implementation in one file versus the run-time CLI usage again source files in their individual locations.

[strarsis]

@kylemh: Well, it never hurts to have some more tests.

[kylemh]

I’ll appreciate it in my fork when I steal it 😂

[TrySound]

Hi @strarsis Could you please rebase?

[kylemh]

Glad you got added to the team @TrySound