gdb/testsuite: split 'maint info sections' tests to a new file

gdb/testsuite: split 'maint info sections' tests to a new file

The next couple of patches are going to add more tests for the 'maint
info sections' command.  Rather than try to jam these tests into the
already quite long gdb.base/maint.c, this commit moves all of the
tests for 'maint info sections' into a new file.

I've updated the tests to make use of some newer testsuite constructs,
like -wrap and $gdb_test_name for gdb_test_multiple, but otherwise the
tests should not have changed with this commit.

gdb/testsuite/ChangeLog:

	* gdb.base/maint-info-sections.exp: New file, content is moved
	from gdb.base/maint.exp and cleaned up to use latest testsuite
	techniques.
	* gdb.base/maint.exp: Tests moved out to
	gdb.base/maint-info-sections.exp.