test ruby and c implementations

.travis.yml

@@ -2,3 +2,6 @@ language: ruby
 rvm:
   - 2.3.3
   - 2.4.0
+env:
+  - BERT_TEST_IMPL=Ruby
+  - BERT_TEST_IMPL=C

lib/bert.rb

@@ -6,12 +6,19 @@ $:.unshift File.join(File.dirname(__FILE__), *%w[.. ext])
 require 'bert/bert'
 require 'bert/types'
 
-begin
-  # try to load the C extension
+case ENV["BERT_TEST_IMPL"]
+when "C"
   require 'bert/c/decode'
-rescue LoadError
-  # fall back on the pure ruby version
+when "Ruby"
   require 'bert/decode'
+else
+  begin
+    # try to load the C extension
+    require 'bert/c/decode'
+  rescue LoadError
+    # fall back on the pure ruby version
+    require 'bert/decode'
+  end
 end
 
 require 'bert/encode'

test/test_helper.rb

@@ -7,4 +7,9 @@ $LOAD_PATH.unshift(File.join(File.dirname(__FILE__), '..', 'ext', 'bert', 'c'))
 
 load 'bert.rb'
 
-puts "Using #{BERT::Decode.impl} implementation."
+if ENV.key?("BERT_TEST_IMPL") && ENV["BERT_TEST_IMPL"] != BERT::Decode.impl
+  raise "Incorrect implementation loaded for value of BERT_TEST_IMPL environment variable! " +
+        "Wanted #{ENV["BERT_TEST_IMPL"]}, but loaded #{BERT::Decode.impl}."
+end
+
+puts "Using #{BERT::Decode.impl} implementation."