Refactor MU blueprint tests: remove ensureDirSync call

[ppcano]

This PR removes the unnecessary call to fs.ensureDirSync('src') on MU blueprint tests.

This change was part of #17212 which was finally solved by #17253.

The tests are failing, but ember-cli/ember-cli#8240 will solve the error, I have created the PR to avoid forgetting this improvement, and I am following the related issue; I will notify when this PR can be finally be merged.

[rwjblue]

Triggered a bunch of blueprint test failures:

29) Blueprint: template
       in addon - module unification
         template foo/bar:
     AssertionError: expected "src/ui/routes/foo/bar/template.hbs" to exist
parent path "src/ui/routes/foo/bar" does not exist.
parent path "src/ui/routes/foo" does not exist.
parent path "src/ui/routes" does not exist.
parent path "src/ui" does not exist.
parent path "src" does not exist.
      at _file (node-tests/blueprints/template-test.js:172:64)
      at emberGenerate.then (node_modules/ember-cli-blueprint-test-helpers/lib/ember-generate-destroy.js:32:17)
      at tryCatcher (node_modules/ember-cli/node_modules/rsvp/dist/rsvp.js:323:19)
      at invokeCallback (node_modules/ember-cli/node_modules/rsvp/dist/rsvp.js:495:31)
      at publish (node_modules/ember-cli/node_modules/rsvp/dist/rsvp.js:481:7)
      at flush (node_modules/ember-cli/node_modules/rsvp/dist/rsvp.js:2402:5)
      at _combinedTickCallback (internal/process/next_tick.js:73:7)
      at process._tickCallback (internal/process/next_tick.js:104:9)

[ppcano]

These test failures will be fixed by ember-cli/ember-cli#8240.

I will update the PR when the fix is included in a new ember-cli release.

[ppcano]

Closing it because ember-cli/ember-cli#8240 does not remove the need to use fs.ensureDirSync('src')