[test_runner] Invoke tests with relative paths.

This makes tests more reproducible, and makes it easier to copy commands between workspaces, or between a failing bot and a local workspace.

Change-Id: Ic8dd10a3540f314a406e5c5b0a23d97032e5d01d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508364
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

tests/standalone/io/process_non_ascii_test.dart

@@ -32,6 +32,7 @@ main() {
     executable,
     []
       ..addAll(Platform.executableArguments)
+      ..add("--packages=" + Platform.packageConfig!)
       ..add(script),
     workingDirectory: nonAsciiDir.path,
     environment: {'DART_CRASHPAD_HANDLER': ''},

tests/standalone/io/regress_7679_test.dart

@@ -41,6 +41,7 @@ main() {
     executable,
     []
       ..addAll(Platform.executableArguments)
+      ..add("--packages=" + Platform.packageConfig!)
       ..add('script.dart'),
     workingDirectory: temp.path,
     environment: {'DART_CRASHPAD_HANDLER': ''},