test: fix generate coverage report & test failure when not supported …

…language with Prismjs (hexojs#271)

* test: fix test failure when not supported language with Prismjs

* test: fix lcov.info is empty

.nycrc.yml

@@ -0,0 +1,2 @@
+# https://github.com/istanbuljs/nyc#common-configuration-options
+all: true

test/prism.spec.js

@@ -77,8 +77,8 @@ describe('prismHighlight', () => {
       '}'
     ].join('\n');
 
-    // Use language: 'plain' to simplify the test
-    const result = prismHighlight(input, { tab: '  ', lang: 'plain' });
+    // Use language: 'plain-text' for not supported language with Prism
+    const result = prismHighlight(input, { tab: '  ', lang: 'plain-text' });
 
     result.should.contains(escapeHTML(input.replace(/\t/g, '  ')));