Add libbej encoder wrapper for C++

Add unit tests and the test data files needed for unit tests.
Move APIs from bej_decoder_test.cpp to a common file, to share those
with bej_encoder_test.cpp

Tested:
Verified that encoding using the wrapper works well at runtime.
Verified that the unit tests pass.

Change-Id: I61b7c17690eb7e7fefd1973a22d4316c6214267e
Signed-off-by: Nikhil Namjoshi <nikhilnamjoshi@google.com>
diff --git a/src/meson.build b/src/meson.build
index 4876d80..b7f0706 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -7,6 +7,7 @@
   'bej_encoder_core.c',
   'bej_encoder_metadata.c',
   'bej_decoder_json.cpp',
+  'bej_encoder_json.cpp',
   include_directories : libbej_incs,
   implicit_include_directories: false,
   version: meson.project_version(),