Enable routine running of regex.linux.utf8 regression test.

Up to now this has just sat there as a test you could invoke via
EXTRA_TESTS, which of course nobody does.  I'm feeling encouraged
because c2e8bd275 hasn't yet broke anything, so let's try making this
run with a suitable guard condition (similar to collate.linux.utf8).

src/test/regress/expected/regex.linux.utf8.out

@@ -1,8 +1,15 @@
 /*
- * This test is for Linux/glibc systems and others that implement proper
- * locale classification of Unicode characters with high code values.
+ * This test is for Linux/glibc systems (conceivably it could be run on
+ * others that implement proper classification of high Unicode characters).
  * It must be run in a database with UTF8 encoding and a Unicode-aware locale.
  */
+SELECT getdatabaseencoding() <> 'UTF8' OR
+       current_setting('lc_ctype') = 'C' OR
+       version() !~ 'linux-gnu'
+       AS skip_test \gset
+\if :skip_test
+\quit
+\endif
 SET client_encoding TO UTF8;
 --
 -- Test the "high colormap" logic with single characters and ranges that

src/test/regress/expected/regex.linux.utf8_1.out

@@ -0,0 +1,11 @@
+/*
+ * This test is for Linux/glibc systems (conceivably it could be run on
+ * others that implement proper classification of high Unicode characters).
+ * It must be run in a database with UTF8 encoding and a Unicode-aware locale.
+ */
+SELECT getdatabaseencoding() <> 'UTF8' OR
+       current_setting('lc_ctype') = 'C' OR
+       version() !~ 'linux-gnu'
+       AS skip_test \gset
+\if :skip_test
+\quit

src/test/regress/parallel_schedule

@@ -94,7 +94,7 @@ test: create_table_like alter_generic alter_operator misc async dbsize misc_func
 # rules cannot run concurrently with any test that creates
 # a view or rule in the public schema
 # collate.*.utf8 tests cannot be run in parallel with each other
-test: rules psql psql_crosstab amutils stats_ext collate.linux.utf8
+test: rules psql psql_crosstab amutils stats_ext collate.linux.utf8 regex.linux.utf8
 
 # run by itself so it can run parallel workers
 test: select_parallel

src/test/regress/sql/regex.linux.utf8.sql

@@ -1,9 +1,17 @@
 /*
- * This test is for Linux/glibc systems and others that implement proper
- * locale classification of Unicode characters with high code values.
+ * This test is for Linux/glibc systems (conceivably it could be run on
+ * others that implement proper classification of high Unicode characters).
  * It must be run in a database with UTF8 encoding and a Unicode-aware locale.
  */
 
+SELECT getdatabaseencoding() <> 'UTF8' OR
+       current_setting('lc_ctype') = 'C' OR
+       version() !~ 'linux-gnu'
+       AS skip_test \gset
+\if :skip_test
+\quit
+\endif
+
 SET client_encoding TO UTF8;
 
 --