test/json/test_helper.rb: Do not add a relative path to $LOAD_PATH

... because it conflicts with test/ruby/test_m17n.rb.

An exception `incompatible character encodings: UTF-8 and UTF-16BE`
occurs when:

* a non-existence relative path is added to $LOAD_PATH,
* ASCII-incompatible encoding is set to default_external, and
* some file is loaded.

```
$LOAD_PATH << "no_existing_dir"
Encoding.default_external = Encoding::UTF_16BE
load "dummy.rb" #=> incompatible character encodings: UTF-8 and UTF-16BE
```

This issue can be actually observed by a combination of out-of-place
build and the following command:

make test-all TESTS="json ruby/m17n -n test_object_inspect_external"

http://ci.rvm.jp/logfiles/brlog.trunk-test-random.20200322-221411

ASCII-incompatible default external encoding assumes that the cwd is the
encoding, and it is attempted to beconcatenated with a non-existence
relative LOAD_PATH UTF-8 string, which causes the exception.

This changeset avoids a relative path.

0 files changed, 0 insertions, 0 deletions