tools: disallow mixed spaces and tabs for indents

Enable eslint rule disallowing mixing tabs and spaces for indentation.
Modify the one file that had been mixing tabs and spaces.

.eslintrc

@@ -65,6 +65,8 @@ rules:
   max-len: [2, 80, 2]
   ## require parens for Constructor
   new-parens: 2
+  ## disallow mixed spaces and tabs for indentation
+  no-mixed-spaces-and-tabs: 2
   ## max 2 consecutive empty lines
   no-multiple-empty-lines: [2, {max: 2}]
   ## no trailing spaces

test/parallel/test-crypto-certificate.js

@@ -22,14 +22,16 @@ var certificate = new crypto.Certificate();
 assert.equal(certificate.verifySpkac(spkacValid), true);
 assert.equal(certificate.verifySpkac(spkacFail), false);
 
-assert.equal(stripLineEndings(certificate.exportPublicKey(spkacValid)
-							  .toString('utf8')),
-			 stripLineEndings(spkacPem.toString('utf8')));
+assert.equal(
+  stripLineEndings(certificate.exportPublicKey(spkacValid).toString('utf8')),
+  stripLineEndings(spkacPem.toString('utf8'))
+);
 assert.equal(certificate.exportPublicKey(spkacFail), '');
 
-assert.equal(certificate.exportChallenge(spkacValid)
-			 .toString('utf8'),
-			 'fb9ab814-6677-42a4-a60c-f905d1a6924d');
+assert.equal(
+  certificate.exportChallenge(spkacValid).toString('utf8'),
+  'fb9ab814-6677-42a4-a60c-f905d1a6924d'
+);
 assert.equal(certificate.exportChallenge(spkacFail), '');
 
 function stripLineEndings(obj) {