test_runner: scrub path of error/warning messages for FXC files

This will avoid annoying diffs every time we regen skips for FXC.

Regen'd all skips with:
```
./tools/run tests -tint ./out/debug/tint.exe -format hlsl -generate-skip
./tools/run tests -tint ./out/debug/tint.exe -format hlsl -use-ir -generate-skip
```

Change-Id: I0b42734afd5500d427743a3c7d2d6e5ecf20b66b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/203397
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: James Price <jrprice@google.com>
diff --git a/test/tint/builtins/gen/literal/quadBroadcast/78129b.wgsl.expected.ir.fxc.hlsl b/test/tint/builtins/gen/literal/quadBroadcast/78129b.wgsl.expected.ir.fxc.hlsl
index c6e1cb8..ae70568 100644
--- a/test/tint/builtins/gen/literal/quadBroadcast/78129b.wgsl.expected.ir.fxc.hlsl
+++ b/test/tint/builtins/gen/literal/quadBroadcast/78129b.wgsl.expected.ir.fxc.hlsl
@@ -17,5 +17,5 @@
 }
 
 FXC validation failure:
-C:\src\dawn\Shader@0x00000214151646B0(3,1-9): error X3000: unrecognized identifier 'float16_t'
+<scrubbed_path>(3,1-9): error X3000: unrecognized identifier 'float16_t'