[Tests] make tests pass in eslint@6

[golopot]

Eslint 6.0.0-rc.0 is released 10 hours ago. This pr mostly fixes these two kinds of errors:

A fatal parsing error occurred: Parsing error: sourceType 'module' is not supported when ecmaVersion < 2015. Consider adding { ecmaVersion: 2015 } to the parser options.

Parsers provided as strings to RuleTester must be absolute paths.

[ljharb]

Can we make eslint 6 explicitly tested in travis.yml, and not allowed to fail?

Also, can we fix appveyor too?

[golopot]

Can we make eslint 6 explicitly tested in travis.yml, and not allowed to fail?

We cannot yet replace eslint@next with eslint@6 because the version eslint@6 does not exists. Maybe just wait until 6 is released?

$ npm i eslint@6
npm ERR! code ETARGET
npm ERR! notarget No matching version found for eslint@6
npm ERR! notarget In most cases you or one of your dependencies are requesting
npm ERR! notarget a package version that doesn't exist.

Also, can we fix appveyor too?

I believe appveyor just randomly fails. At least it passed in my fork. It seemed that the test fails when a downloaded package tarball is corrupted.

[ljharb]

@golopot true, but we can use @^6.0.0-0.

[golopot]

Appveyor still fails at npm install.

Maybe this pr can wait until the release of eslint 6, which I think will be in 2 weeks.

[petersendidit]

I believe >6.0.0-rc would work for installing version 6 before its out of a prerelease version.

[ljharb]

@petersendidit yes, but then that would include v7 as well in the future. ^6.0.0-0 is the proper semver range.