Test case for BZ 25065

Running a GDB with the fix for BZ 25065 should cause these new tests
to all pass.

When run against a GDB without the fix, there will be 2 unresolved
testcases.  This is what I see in the gdb.sum file when I try it using
a GDB without the fix:

ERROR: GDB process no longer exists
UNRESOLVED: gdb.dwarf2/imported-unit.exp: ptype main::Foo
ERROR: Couldn't send ptype main::foo to GDB.
UNRESOLVED: gdb.dwarf2/imported-unit.exp: ptype main::foo

These are "unresolved" versus outright failures due to the fact that
GDB dies (segfaults) during the running of the test.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/imported-unit.exp: New file.
	* gdb.dwarf2/imported-unit.c: New file.

Change-Id: I073fe69b81bd258951615f752df8e95b6e33a271