llvm split-file

[Johan]

Hi all,
   Have a look at this LLVM test tool: split-file.  
https://reviews.llvm.org/D83834?id=282671

It makes it much more convenient to make testcases that span 
multiple files (like import files), but also grouping tests that 
require independent files but are closely related (instead of 
creating separate test files like "test_feature_1.d"  
"test_feature_2.d"  "test_feature_3.d"...)

I'll submit a PR to add it to LDC later.

cheers,
   Johan