Update from .t to .rakutest

Avoid duplicating list of preferred AND historic extensions here,
refer to existing list instead.

Author: coke

doc/Language/modules.pod6

@@ -51,12 +51,8 @@ L<synopsis S11|https://design.raku.org/S11.html#Units> says: Confusing? Yes it
 is.> , a C<t> directory for tests, and possibly a C<bin> directory for
 executable programs and scripts.
 
-Source files generally use the C<.rakumod> extension, and scripts or
-executables use the C<.raku>. Test files use the C<.rakutest> (or C<.t>)
-extension. Files which contain documentation use the C<.rakudoc> extension N«The
-old C<.p6> (or C<.pl6>), C<.pm6> (or C<.pm> which is also supported, but
-discouraged) and C<.pod6> extensions will continue to be supported for 6.e
-and marked as B<deprecated> in 6.f».
+See L<Language/filename-extensions> for current and historic
+extensions for various file types.
 
 =head1 X<Loading and basic importing|Language,compunit>
 
@@ -632,7 +628,7 @@ control, but it can slow down the installation of popular distributions.
 An alternative to placing the C<meta-ok> test in an extended test directory, but
 to ensure that it is only run when a developer or maintainer wants to, is to
 make the test dependent on an environment variable, e.g., in
-C<t/99-author-test.t> there is the following code
+C<t/99-author-test.rakutest> there is the following code
 
 =begin code
 use Test;
@@ -706,7 +702,7 @@ Vortex-TotalPerspective/
 └── bin
 │    └── vortex
 └── t
-    └── basic.t
+    └── basic.rakutest
 =end code
 
 If your project contains other modules that help the main module do
@@ -785,7 +781,7 @@ you should put the license's name in the license field. For more details see
 L<https://design.raku.org/S22.html#license>.
 
 =item If you don't yet have any tests, you can leave out the C<t>
-directory and C<basic.t> file for now. For more information on how to write
+directory and C<basic.rakutest> file for now. For more information on how to write
 tests (for now), you might have a look at how other modules use
 C<Test>.
 

doc/Language/testing.pod6

@@ -70,7 +70,7 @@ Tests can be run individually by specifying the test filename on the
 command line:
 
 =for code :lang<shell>
-$ raku t/test-filename.t
+$ raku t/test-filename.rakutest
 
 To run all tests in the directory recursively,
 L<prove6|https://modules.raku.org/dist/App::Prove6> application can
@@ -98,7 +98,7 @@ To abort the test suite upon first failure, set the
 C<RAKU_TEST_DIE_ON_FAIL> environmental variable:
 
 =for code :lang<shell>
-$ RAKU_TEST_DIE_ON_FAIL=1 raku t/test-filename.t
+$ RAKU_TEST_DIE_ON_FAIL=1 raku t/test-filename.rakutest
 
 The same variable can be used within the test file. Set it before loading
 the C<Test> module:

doc/Type/Test.pod6

@@ -699,7 +699,7 @@ The result from the test code above will be something like:
 =begin code :lang<TAP>
 not ok 1 - my-custom-pi # TODO not yet precise enough
 # Failed test 'my-custom-pi'
-# at test-todo.t line 7
+# at test-todo.rakutest line 7
 # expected: '3.14159265358979'
 #      got: '3'
 =end code