test/error.import_folder.jsonnet has different output depending on OS

[tamsky]

Runtime platform OSX 10.10.3:

# ../jsonnet --gc-min-objects 1 --gc-growth-trigger 1 error.import_folder.jsonnet
STATIC ERROR: lib:1:1: Unexpected end of file.

this error is different from the linux runtime error, https://github.com/google/jsonnet/blob/master/test_suite/error.import_empty.jsonnet.golden
so the test fails.

Suggested fix to work on multiple OS platforms might be to insert platform name (eg error.import_folder.jsonnet.darwin.golden) and test for existence of a platform-specific file within run_tests.sh.