issue with directory import test

https://github.com/google/jsonnet/blob/2caedc178adaf5de78fe7b8d3273a047bcf412f9/core/libjsonnet.cpp#L190
On some systems (FreeBSD) an attempt to import a directory with `try_path()` won't lead to a failure, and the error will be detected only by the lexer. For this reason one of the tests fails with something like this
```
FAIL (regex mismatch): ../jsonnet --ext-str var1=test --ext-code var2='{x:1,y:2}' error.import_folder.jsonnet
This run's output:
STATIC ERROR: lib:1:2: Could not lex the character code 210
```