c++: improve C++ testsuite default versions

c++: improve C++ testsuite default versions

I wanted to add more cases to the setting of std_list in g++-dg.exp, but
didn't want to do a full scan through the file for each case.  So this patch
improves that in two ways: first, by extracting all interesting lines on a
single pass; second, by generating the list more flexibly: now we test every
version mentioned explicitly in the testcase, plus a few more if fewer than
three are mentioned.

This also lowers changes from testing four to three versions for most
testcases: the current default and the earliest and latest versions.  This
will reduce testing of C++14 and C++20 modes, and increase testing of C++26
mode.  C++ front-end developers are encouraged to set the
GXX_TESTSUITE_STDS environment variable to test more modes.

gcc/testsuite/ChangeLog:

	* lib/gcc-dg.exp (get_matching_lines): New.
	* lib/g++-dg.exp: Improve std_list selection.