Fix collate.windows.win1252 test.

Do not rely on the OS recognizing a particular locale; find the right
locale by querying the "en_US" collation.

Author: Alexander Lakhin
Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/ae73f6f5-8221-c112-4640-5cda812a69de@gmail.com

src/test/regress/expected/collate.windows.win1252.out

@@ -992,7 +992,13 @@ drop type textrange_c;
 drop type textrange_en_us;
 -- nondeterministic collations
 -- (not supported with libc provider)
-CREATE COLLATION ctest_det (locale = 'en_US', deterministic = true);
+do $$
+BEGIN
+  EXECUTE 'CREATE COLLATION ctest_det (locale = ' ||
+          quote_literal((SELECT collcollate FROM pg_collation WHERE
+          collname = 'en_US')) || ', deterministic = true);';
+  END
+$$;
 CREATE COLLATION ctest_nondet (locale = 'en_US', deterministic = false);
 ERROR:  nondeterministic collations not supported with this provider
 -- cleanup

src/test/regress/sql/collate.windows.win1252.sql

@@ -400,8 +400,13 @@ drop type textrange_en_us;
 
 -- nondeterministic collations
 -- (not supported with libc provider)
-
-CREATE COLLATION ctest_det (locale = 'en_US', deterministic = true);
+do $$
+BEGIN
+  EXECUTE 'CREATE COLLATION ctest_det (locale = ' ||
+          quote_literal((SELECT collcollate FROM pg_collation WHERE
+          collname = 'en_US')) || ', deterministic = true);';
+  END
+$$;
 CREATE COLLATION ctest_nondet (locale = 'en_US', deterministic = false);