Provide local paths for file tests. (#2830)

The intent of this change is that instead of paths looking like `explorer/testdata/foo/bar.carbon` (repo-relative), they're now just `bar.carbon` (local). The consequence is that paths should be a bit more durable in various environments, and just paths should be shorter and easier to read.

The explorer's prelude is an exception to this since it comes from data, rather than being the test target. Due to the change in approaches, it needs the regex again.

Uses #2829

testing/scripts/autoupdate_testdata_base.py

@@ -87,7 +87,7 @@ def parse_args() -> ParsedArgs:
     parser.add_argument(
         "--line_number_pattern",
         metavar="PATTERN",
-        default=r"(?P<prefix>/(?P<filename>\w+\.carbon):)"
+        default=r"(?P<prefix>(?P<filename>\w+\.carbon):)"
         r"(?P<line>\d+)(?P<suffix>(?:\D|$))",
         help="A regular expression which matches line numbers to update as its "
         "only group. Capture groups 'prefix', 'line', and 'suffix' are "
@@ -304,13 +304,16 @@ def get_matchable_test_output(
     """Runs the autoupdate command and returns the output lines."""
     # Run the autoupdate command to generate output.
     # (`bazel run` would serialize)
-    autoupdate_cmd = TOOLS[tool].replace("//", "./bazel-bin/").replace(":", "/")
+    autoupdate_cmd = Path.cwd().joinpath(
+        TOOLS[tool].replace("//", "./bazel-bin/").replace(":", "/")
+    )
     p = subprocess.run(
-        [autoupdate_cmd] + autoupdate_args + [test],
+        [str(autoupdate_cmd)] + autoupdate_args + [Path(test).name],
         env={"LLVM_SYMBOLIZER_PATH": llvm_symbolizer},
         stdout=subprocess.PIPE,
         stderr=subprocess.PIPE,
         encoding="utf-8",
+        cwd=str(Path(test).parent),
     )
 
     out_lines = label_output("STDOUT:", p.stdout)
@@ -324,10 +327,7 @@ def get_matchable_test_output(
             # `lit` uses full paths to the test file, so use a regex to ignore
             # paths when used.
             line = line.replace(test, f"{{{{.*}}}}/{test}")
-            line = bazel_runfiles.sub("{{.*}}/", line)
-        else:
-            # When not using `lit`, the runfiles path is removed.
-            line = bazel_runfiles.sub("", line)
+        line = bazel_runfiles.sub("{{.*}}/", line)
 
         for line_matcher, before, after in extra_check_replacements:
             if line_matcher.match(line):