[test][DebugInfo] Fix location of test build artifacts (llvm#165349)

The test added in llvm#161067 writes artifacts to the current dir, i.e.
`test.o` / `test.dwo` / `test.dwp`, which might not be writeable. Tests
should use `%t` for test artifact location, i.e. `%t.o` / `%t.dwo` /
`%t.dwp` However, since `"test.dwo"` is part of the assembly source file
used as a test input, and that's not something lit will substitute, that
typical approach doesn't work. We can instead ensure the output is in a
good location by running `cd %t` (after setting it up).

Author: rupprecht

llvm/test/tools/llvm-dwarfdump/X86/type_units_split_dwp_v4.s

@@ -1,6 +1,12 @@
 ## This test uses TU index for type parsing in dwp and makes sure the DWARF4 type is
 ## successfully retrieved.
 
+## cd to a unique dir so we can refer to the file as just "test.dwo" in the
+## assembly test input below.
+# RUN: rm -rf %t
+# RUN: mkdir %t
+# RUN: cd %t
+
 # RUN: llvm-mc %s --split-dwarf-file=test.dwo -filetype obj -triple x86_64 -o test.o
 # RUN: llvm-dwp -e test.o -o test.dwp
 # RUN: llvm-dwarfdump test.dwp | FileCheck %s