Add test case for gdb 10 crash

Add test case for gdb 10 crash

PR gdb/27743 points out a gdb crash when expanding partial symtabs,
where one of the compilation units uses DW_TAG_imported_unit.

This crash happens for gdb 10, but not git trunk.  This patch pulls
over the new test case only.

gdb/testsuite/ChangeLog
2021-04-26  Tom Tromey  <tromey@adacore.com>

	PR gdb/27743:
	* gdb.dwarf2/imported-unit-bp.exp: New file.
	* gdb.dwarf2/imported-unit-bp-main.c: New file.
	* gdb.dwarf2/imported-unit-bp-alt.c: New file.