As work towards supporting multiple diagnostic outputs (where each
output has its own pretty_printer), avoid referencing dc.m_printer
throughout the selftests of diagnostic-show-locus.cc.  Instead
have test_diagnostic_context::test_show_locus return the result
buffer, hiding the specifics of which printer is in use in such
test cases.

No functional change intended.

gcc/ChangeLog:
	PR other/116613
	* diagnostic-show-locus.cc
	(selftest::test_diagnostic_show_locus_unknown_location): Move call
	to dc.test_show_locus into ASSERT_STREQ, and compare against its
	result, rather than explicitly using dc.m_printer.
	(selftest::test_one_liner_simple_caret): Likewise.
	(selftest::test_one_liner_no_column): Likewise.
	(selftest::test_one_liner_caret_and_range): Likewise.
	(selftest::test_one_liner_multiple_carets_and_ranges): Likewise.
	(selftest::test_one_liner_fixit_insert_before): Likewise.
	(selftest::test_one_liner_fixit_insert_after): Likewise.
	(selftest::test_one_liner_fixit_remove): Likewise.
	(selftest::test_one_liner_fixit_replace): Likewise.
	(selftest::test_one_liner_fixit_replace_non_equal_range):
	Likewise.
	(selftest::test_one_liner_fixit_replace_equal_secondary_range):
	Likewise.
	(selftest::test_one_liner_fixit_validation_adhoc_locations):
	Likewise.
	(selftest::test_one_liner_many_fixits_1): Likewise.
	(selftest::test_one_liner_many_fixits_2): Likewise.
	(selftest::test_one_liner_labels): Likewise.
	(selftest::test_one_liner_simple_caret_utf8): Likewise.
	(selftest::test_one_liner_multiple_carets_and_ranges_utf8):
	Likewise.
	(selftest::test_one_liner_fixit_insert_before_utf8): Likewise.
	(selftest::test_one_liner_fixit_insert_after_utf8): Likewise.
	(selftest::test_one_liner_fixit_remove_utf8): Likewise.
	(selftest::test_one_liner_fixit_replace_utf8): Likewise.
	(selftest::test_one_liner_fixit_replace_non_equal_range_utf8):
	Likewise.
	(selftest::test_one_liner_fixit_replace_equal_secondary_range_utf8):
	Likewise.
	(selftest::test_one_liner_fixit_validation_adhoc_locations_utf8):
	Likewise.
	(selftest::test_one_liner_many_fixits_1_utf8): Likewise.
	(selftest::test_one_liner_many_fixits_2_utf8): Likewise.
	(selftest::test_one_liner_labels_utf8): Likewise.
	(selftest::test_one_liner_colorized_utf8): Likewise.
	(selftest::test_add_location_if_nearby): Likewise.
	(selftest::test_diagnostic_show_locus_fixit_lines): Likewise.
	(selftest::test_overlapped_fixit_printing): Likewise.
	(selftest::test_overlapped_fixit_printing_utf8): Likewise.
	(selftest::test_overlapped_fixit_printing_utf8): Likewise.
	(selftest::test_overlapped_fixit_printing_2): Likewise.
	(selftest::test_fixit_insert_containing_newline): Likewise.
	(selftest::test_fixit_insert_containing_newline_2): Likewise.
	(selftest::test_fixit_replace_containing_newline): Likewise.
	(selftest::test_fixit_deletion_affecting_newline): Likewise.
	(selftest::test_tab_expansion): Likewise.
	(selftest::test_escaping_bytes_1): Likewise.
	(selftest::test_escaping_bytes_2): Likewise.
	(selftest::test_line_numbers_multiline_range): Likewise.
	* selftest-diagnostic.cc
	(selftest::test_diagnostic_context::test_show_locus): Return the
	formatted text of m_printer.
	* selftest-diagnostic.h
	(selftest::test_diagnostic_context::test_show_locus): Convert
	return type from void to const char *.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>