Enable integration tests with environment var

I think this makes more sense then running them via a separate command,
since that command would only make sense for atom-core, unlike the
current 'run-package-specs' command, which works for any atom package.

This way, they won't run by default, but you can opt in to running them
on the command line by setting an env var, or in the spec runner by
temporarily editing the code, like we do for focused tests anyway.

spec/integration/startup-spec.coffee

@@ -1,3 +1,8 @@
+# These tests are excluded by default. To run them from the command line:
+#
+# ATOM_INTEGRATION_TESTS_ENABLED=true apm test
+return unless process.env.ATOM_INTEGRATION_TESTS_ENABLED
+
 os = require "os"
 fs = require "fs"
 path = require "path"
@@ -12,18 +17,16 @@ SocketPath = path.join(os.tmpdir(), "atom-integration-test.sock")
 ChromeDriverPort = 9515
 
 describe "Starting Atom", ->
-  if spawnSync("which", ["chromedriver"]).status isnt 0
-    console.log "Skipping integration tests because the `chromedriver` executable was not found."
-    return
-
   [chromeDriver, driver, tempDirPath] = []
 
   beforeEach ->
     tempDirPath = temp.mkdirSync("empty-dir")
-    chromeDriver = spawn "chromedriver", ["--verbose", "--port=#{ChromeDriverPort}"]
 
-    # Uncomment to see chromedriver debug output
-    # chromeDriver.stderr.on "data", (d) -> console.log(d.toString())
+    waitsFor "chromedriver to start", (done) ->
+      chromeDriver = spawn "chromedriver", ["--verbose", "--port=#{ChromeDriverPort}"]
+      chromeDriver.on "error", (error) ->
+        throw new Error("chromedriver failed to start: #{error.message}")
+      chromeDriver.stderr.on "data", -> done()
 
   afterEach ->
     waitsForPromise -> driver.quit().thenFinally(-> chromeDriver.kill())