Make ALTER TABLE ... SET ACCESS METHOD logic easier to read

[jeltz]

This also cleans up the tests and adds a couple of tests for the DEFAULT which I wanted myself to avoid regressions when doing this refactor.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.12%. Comparing base (57ac8c8) to head (d118b58).

❌ Your project status has failed because the head coverage (78.12%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           TDE_REL_17_STABLE     #257      +/-   ##
=====================================================
- Coverage              78.13%   78.12%   -0.01%     
=====================================================
  Files                     22       22              
  Lines                   2488     2487       -1     
  Branches                 393      393              
=====================================================
- Hits                    1944     1943       -1     
  Misses                   466      466              
  Partials                  78       78              

Components	Coverage Δ
access	78.67% <ø> (ø)
catalog	86.06% <ø> (ø)
common	92.50% <ø> (ø)
encryption	71.90% <ø> (ø)
keyring	72.07% <ø> (ø)
src	53.64% <100.00%> (-0.16%)	⬇️
smgr	98.01% <ø> (ø)
transam	∅ <ø> (∅)

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