chore: switch from memfs to fstest.MapFS

[jawnsy]

• Switch from memfs to built-in fstest.MapFS
• Ensure httptest servers are closed during test cleanup
• Swap ordering of expected/actual values in assertion functions
• Use http.StatusOK constants for status codes
• Add a 1-second context timeout for each request
• Use the test httptest.Server.Client() so that we can handle
  TLS server certificates if desired

[codecov]

Codecov Report

Merging #327 (ec71732) into main (37bcfeb) will increase coverage by 3.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #327      +/-   ##
==========================================
+ Coverage   63.61%   67.49%   +3.87%     
==========================================
  Files          69      140      +71     
  Lines         786     7407    +6621     
  Branches       77       77              
==========================================
+ Hits          500     4999    +4499     
- Misses        271     1897    +1626     
- Partials       15      511     +496     

Flag	Coverage Δ
unittest-go-macos-latest	66.09% <ø> (?)
unittest-go-ubuntu-latest	67.37% <ø> (?)
unittest-go-windows-latest	65.69% <ø> (?)
unittest-js	63.61% <ø> (ø)

Impacted Files	Coverage Δ
pty/start_other.go	66.66% <0.00%> (ø)
httpmw/workspaceparam.go	76.47% <0.00%> (ø)
coderd/projectversion.go	55.22% <0.00%> (ø)
provisioner/terraform/parse.go	77.77% <0.00%> (ø)
coderd/users.go	58.76% <0.00%> (ø)
codersdk/client.go	58.46% <0.00%> (ø)
codersdk/workspaces.go	64.03% <0.00%> (ø)
peer/channel.go	82.45% <0.00%> (ø)
provisioner/terraform/provision.go	71.62% <0.00%> (ø)
httpmw/organizationparam.go	69.81% <0.00%> (ø)
... and 61 more

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[kylecarbs]

Nice! Another dependency bites the dust!