WIP: start converting user management views' tests to pytest format

[rebeccacremona]

This work was started during the Q3 housekeeping cycle, but was not finished.

Still to do:

• [ ] test_org_export_user_list
• [ ] test_sponsored_user_export_user_list
• [ ] test_create_and_delete_user
• [ ] All the tests in "### ADDING NEW USERS TO ORGANIZATIONS ###"
• [ ] All the tests in "### ADDING EXISTING USERS TO ORGANIZATIONS ###"
• [ ] All the tests in "### VOLUNTARILY LEAVING ORGANIZATIONS ###"
• [ ] All the tests in "### REMOVING USERS FROM ORGANIZATIONS ###"
• [ ] All the tests in "### ADDING NEW USERS TO REGISTRARS AS SPONSORED USERS ###"
• [ ] All the tests in "### ADDING EXISTING USERS TO REGISTRARS AS SPONSORED USERS ###"
• [ ] All the tests in "### TOGGLING THE STATUS OF SPONSORSHIPS ###"
• [ ] All the tests in "### ADDING NEW USERS TO REGISTRARS AS REGISTRAR USERS) ###"
• [ ] All the tests in "### ADDING EXISTING USERS TO REGISTRARS ###"
• [ ] All the tests in "### REMOVING REGISTRAR USERS FROM REGISTRARS ###"
• [ ] All the tests in "### ADDING NEW USERS AS ADMINS ###"
• [ ] All the tests in "### ADDING EXISTING USERS AS ADMINS ###"
• [ ] All the tests in "### REMOVING USERS AS ADMINS ###"

Not in scope:

• Any tests of filters: those are going to be a pain to convert, as long as JSON fixtures are still being installed.
• "### SIGNUP" (should be their own file)
• "### PASSWORD RESETS ###" (should be elsewhere)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.

Project coverage is 69.73%. Comparing base (589f291) to head (b934d0b). Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
perma_web/conftest.py	69.23%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #3621 +/- ## =========================================== - Coverage 69.73% 69.73% -0.01% =========================================== Files 54 54 Lines 7326 7338 +12 =========================================== + Hits 5109 5117 +8 - Misses 2217 2221 +4 ```

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