testing/README Document use of regexp filter for testing

testing/README

@@ -53,15 +53,38 @@ See ../mk/default.mk for details.
 
 The 'dirty' targets are for recompiling without cleaning and
 rebuilding everything.  This usually speeds up the recompilation
-considerably.
+considerably.  Note that this speed up comes to the price of possibly
+weird errors due to the unclean build.
 
-The 'dirty' target is called test-dirty.
+The dirty target for testing is called ~test-dirty~.
 
 #+BEGIN_SRC sh :dir (expand-file-name "..") :results silent
 make test-dirty
 #+END_SRC
 
-Note that the outcome may /not/ be in perfect shape.
+** Select tests by regexp
+
+Variable ~BTEST_RE~ can be set to limit the tests which are performed.
+~BTEST_RE~ is interpreted as regexp.
+
+Example:
+
+#+begin_src shell
+make BTEST_RE='test-.*-inlinetask' test-dirty
+#+end_src
+
+yields
+
+#+begin_example
+...
+selected tests: test-.*-inlinetask
+Running 2 tests (2017-12-28 15:04:45+0100)
+   passed  1/2  test-org-export/handle-inlinetasks
+   passed  2/2  test-org-inlinetask/goto-end
+
+Ran 2 tests, 2 results as expected (2017-12-28 15:04:45+0100)
+...
+#+end_example
 
 * Interactive testing from within Emacs