Neg tests check files for // error markers (rebased and updated)

[VladimirNik]

Rebased and updated version of #720

Differences between this and previous pull request (#720):

1. All neg tests are updated in accordance with new checks;
2. Starting from 73fad44 CompilerTest doesn't extend DottyTest and we cannot use DottyTest#ctx in CompilerTest. Because of that:

val allFiles = CompilerCommand.distill(allArgs)(ctx).arguments

is changed to:

val allFiles = (allArgs filter {
  arg => !arg.startsWith("-") && (arg.endsWith(".scala") || arg.endsWith(".java"))
}).toList

Review by @odersky @DarkDimius

[DarkDimius]

is changed to:

AFAIK all special scalac options start from -, so I'd add filter that filenames cannot start from -

[VladimirNik]

@DarkDimius '-' filtering updated

[DarkDimius]

is this a pos test or a neg test.
If some option is needed to make test fail, please specify it.

[VladimirNik]

This test is no longer neg test. Comment is removed

[DarkDimius]

LGTM