selftest.h: add class line_table_test

selftest.h: add class line_table_test

input.c has a fixture class for running each selftest with a fresh
line_table, and logic for looping over various interesting line_table
test cases.

This patch exposes the above in selftest.h so that such
location-handling tests can be written in other files, renaming the
class from temp_line_table to line_table_test.

Also, the patch moves the stored line table ptr from being a member of
the test class to being a global GC-root, to avoid it being collected
if the GC runs during such a test.

gcc/ChangeLog:
	* input.c (saved_line_table): New global.
	(class selftest::temp_line_table): Rename to line_table_test and
	move declaration to selftest.h, and drop field m_old_line_table.
	(selftest::temp_line_table::temp_line_table): Rename ctor to...
	(selftest::line_table_test::line_table_test): ...this.  Add a
	default ctor.  Store current value of line_table within
	saved_line_table.
	(selftest::temp_line_table::~temp_line_table): Rename dtor to...
	(selftest::line_table_test::~line_table_test): ...this, and
	restore line_table from the saved_line_table, rather than
	m_old_line_table.
	(selftest::test_accessing_ordinary_linemaps): Update for above
	renaming.
	(selftest::test_lexer): Likewise.
	(struct selftest::lexer_test): Likewise.
	(selftest::lexer_test::lexer_test): Likewise.
	(selftest::input_c_tests): Move the looping over test cases from
	here into...
	(selftest::for_each_line_table_case): New function.
	* input.h (saved_line_table): New decl.
	* selftest.h (struct selftest::line_table_case): New forward decl.
	(class selftest::line_table_test): New class, moved here from
	selftest::temp_line_table in input.c, and renamed.
	(selftest::for_each_line_table_case): New decl.

From-SVN: r239580