Refactor ssh_keys specs in spec/controllers/users_controller_spec.rb

The following discussions from !48991 (merged) should be addressed:

suggestion (non-blocking): Please add a newline between the execution and the expectation phase.
          get :ssh_keys, params: { username: user.username }

          expect(response.body).not_to match(/dummy@gitlab.com/)

@dbalexandre started a discussion:

suggestion (non-blocking): Please add a newline between the execution and the expectation phase.
          get :ssh_keys, params: { username: user.username }

          expect(response.body).not_to match(/dummy@gitlab.com/)

@dbalexandre started a discussion:
```
    context "non existent user" do
```
@dbalexandre started a discussion:
```
    context "user with no keys" do
```
@dbalexandre started a discussion:
```
    context "user with keys" do
```
@dbalexandre started a discussion:
```
      context "while signed in" do
```
@dbalexandre started a discussion:
```
      context 'when logged out' do
```
@dbalexandre started a discussion:

Both contexts while signed in and when logged out have the same tests. What do you think to extract them into shared examples?

Edited Dec 22, 2020 by Takuya Noguchi