test: fix in-test comments related to obsolescence

The `ui.prevent-unstable` option never made it into core. It always behaves
this way when obsolescence feature is enabled.

See changesets c5bd753c5bc6, bacf55bd8f90 and 0f5a0a2073a8 for details.

tests/test-commit-amend.t

@@ -500,7 +500,7 b' Test that amend does not make it easy to'
   $ hg id
   b99e5df575f7 (a) tip
 
-Test ui.prevent-unstable
+Test that rewriting leaving instability behind is allowed
 ---------------------------------------------------------------------
 
   $ hg up '.^'

tests/test-histedit-obsolete.t

@@ -143,8 +143,8 b' Check that histedit respect phases'
   [255]
 
 
-+Test ui.prevent-unstable option
-+------------------------------------
+Test that rewriting leaving instability behind is allowed
+---------------------------------------------------------------------
 
   $ hg up '.^'
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved

tests/test-rebase-obsolete.t

@@ -279,8 +279,8 b' not be rebased.'
   32af7686d403cf45b5d95f2d70cebea587ac806a cf44d2f5a9f4297a62be94cbdd3dff7c7dc54258 0 {'date': '*', 'user': 'test'} (glob)
   42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 7c6027df6a99d93f461868e5433f63bde20b6dfb 0 {'date': '*', 'user': 'test'} (glob)
 
-Test ui.prevent-unstable option
-------------------------------------
+Test that rewriting leaving instability behind is allowed
+---------------------------------------------------------------------
 
   $ hg log -r 'children(8)'
   9:cf44d2f5a9f4 D (no-eol)