[testing] Make sharding deterministic

The current sharding logic hashes the absolute file path to a test file.
This makes sharding more or less non-deterministic across different
machines given that it depends on the build output directory location
on disk.

This makes debugging bot failures with shards impossible.

This CL makes the sharding deterministic by hashing the test file name
relative to the directory and uses '/' consistently for the hashed
string so sharding stays the same on all platforms.

R=kimanh@chromium.org

Bug: None
No-Tree-Checks: True
Change-Id: I58dea4ff79eb9135ef60bb861884cf2765a6f336
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7660459
Reviewed-by: Danil Somsikov <dsv@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Reviewed-by: Kim-Anh Tran <kimanh@chromium.org>
Commit-Queue: Kim-Anh Tran <kimanh@chromium.org>

Author: szuend

test/conductor/test_config.ts

@@ -143,7 +143,10 @@ export function loadTests(testDirectory: string, filename = 'tests.txt') {
                     .filter(t => t.length > 0)
                     .map(t => path.normalize(path.join(testDirectory, t)))
                     .filter(t => TestConfig.tests.some((spec: string) => t.startsWith(spec)))
-                    .filter(t => shardFilter(TestConfig, t));
+                    // To keep sharding deterministic, use the relative path from the test directory, NOT the
+                    // absolute file path on disk. Also replace backward slashes with forward slashes so sharding stays
+                    // the same across windows, linux and mac.
+                    .filter(t => shardFilter(TestConfig, path.relative(testDirectory, t).replaceAll('\\', '/')));
 
   if (TestConfig.shuffle) {
     for (let i = tests.length - 1; i > 0; i--) {