Refactor registration tests, remove hard-coded username validation

[CicadaCinema]

Pull Request Checklist

• I have added Go unit tests or Complement integration tests for this PR or I have justified why this PR doesn't need tests
• I have already signed off privately

This PR is in preparation for #3137 and removes the hard-coded username validation (previously only dependent on forceEmpty).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7863a40) 65.71% compared to head (e203b01) 63.91%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3138      +/-   ##
==========================================
- Coverage   65.71%   63.91%   -1.81%     
==========================================
  Files         507      506       -1     
  Lines       57291    57449     +158     
==========================================
- Hits        37651    36717     -934     
- Misses      15830    16895    +1065     
- Partials     3810     3837      +27     

Flag	Coverage Δ
unittests	49.51% <ø> (+<0.01%)	⬆️

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[CicadaCinema]

Are these known flaky tests? On my machine I get:

Are We Synapse Yet?
===================

Non-Spec APIs: 76% (13/17 tests)
--------------
Non-Spec API             :  80% (4/5 tests)
MSCs                     :  78% (7/9 tests)
Unknown API (no group specified):  67% (2/3 tests)

Client-Server APIs: 93% (578/621 tests)
-------------------
Registration             :  91% (31/34 tests)
Login                    :  63% (12/19 tests)
Misc CS APIs             : 100% (1/1 tests)
Profile                  : 100% (6/6 tests)
Devices                  : 100% (12/12 tests)
Presence                 : 100% (15/15 tests)
Create Room              : 100% (13/13 tests)
Sync API                 :  90% (71/79 tests)
Room Membership          : 100% (11/11 tests)
Room State APIs          : 100% (15/15 tests)
Public Room APIs         : 100% (7/7 tests)
Room Aliases             : 100% (15/15 tests)
Joining Rooms            : 100% (8/8 tests)
Leaving Rooms            : 100% (1/1 tests)
Inviting users to Rooms  : 100% (14/14 tests)
Banning users            : 100% (5/5 tests)
Sending events           : 100% (2/2 tests)
Getting events for Rooms : 100% (10/10 tests)
Typing API               : 100% (5/5 tests)
Receipts                 : 100% (2/2 tests)
Read markers             : 100% (1/1 tests)
Media APIs               :  95% (20/21 tests)
Capabilities API         : 100% (2/2 tests)
Logout                   : 100% (4/4 tests)
Push APIs                : 100% (53/53 tests)
Account APIs             : 100% (10/10 tests)
V1 CS APIs               : 100% (1/1 tests)
Ephemeral Events         :   0% (0/1 tests)
Power Levels             : 100% (6/6 tests)
Redaction                : 100% (6/6 tests)
Third-Party ID APIs      :  16% (3/19 tests)
Guest APIs               :  92% (22/24 tests)
Room Auth                : 100% (17/17 tests)
Context APIs             : 100% (4/4 tests)
Room Upgrade APIs        : 100% (21/21 tests)
Room Versions            :  99% (82/83 tests)
Device Keys              :  94% (17/18 tests)
Device Key Backup        : 100% (10/10 tests)
Cross-signing Keys       : 100% (8/8 tests)
Tagging APIs             : 100% (8/8 tests)
OpenID API               : 100% (3/3 tests)
Send-to-Device APIs      : 100% (10/10 tests)
Server Admin API         : 100% (1/1 tests)
Ignore Users             : 100% (3/3 tests)
User Directory APIs      :  70% (7/10 tests)
Enforced canonical JSON  : 100% (3/3 tests)

Federation APIs: 100% (115/115 tests)
----------------
State APIs               : 100% (19/19 tests)
Invite API               : 100% (11/11 tests)
Device Key APIs          : 100% (9/9 tests)
Send-to-Device APIs      : 100% (2/2 tests)
Key API                  : 100% (6/6 tests)
Query API                : 100% (5/5 tests)
send_join API            : 100% (10/10 tests)
make_join API            : 100% (3/3 tests)
Auth                     : 100% (19/19 tests)
room versions            : 100% (7/7 tests)
Federation API           : 100% (13/13 tests)
get_missing_events API   : 100% (3/3 tests)
Backfill API             : 100% (5/5 tests)
send_leave API           : 100% (1/1 tests)
Public Room API          : 100% (2/2 tests)

Application Services APIs: 76% (19/25 tests)
--------------------------
Application Services API :  76% (19/25 tests)

+ exit 0
ubuntu@dendrite-dev:~/dendrite$ git branch
  main
  numeric-username-restriction
* refactor-registration-tests