ci: Only run tests for modified code on presubmits.

Towards #1690

.kokoro/main.ps1

@@ -23,10 +23,24 @@ try {
 
     # Import secrets:
     .\.kokoro-windows\Import-Secrets.ps1
-
+
+    # The list of changed subdirectories.
+    git config --global --add safe.directory /tmpfs/src/github/dotnet-docs-samples
+    $changedDirs = ($(git diff main --name-only | cut -d/ -f 1 | uniq))
+
     # The list of all subdirectories.
     $allDirs = Get-ChildItem | Where-Object {$_.PSIsContainer} | Select-Object -ExpandProperty Name
 
+    # If no dirs have changed we run everything since we are most likely on CI.
+    if ($changedDirs.Count -gt 0)
+    {
+        $testDirs = $changedDirs
+    }
+    else
+    {
+        $testDirs = $allDirs
+    }
+
     # There are too many tests to run in a single Kokoro job.  So, we split
     # the tests into groups.  Each Kokoro job runs one group.
 
@@ -38,17 +52,20 @@ try {
         $false   # 3: Everything starting from s to z.
     )
 
-    $groups[0] = $allDirs
-    $groups[1] = $allDirs | Where-Object { ($_.Substring(0, 1).CompareTo("a") -ge 0) -and ($_.Substring(0, 1).CompareTo("e") -le 0) }
-    $groups[2] = $allDirs | Where-Object { ($_.Substring(0, 1).CompareTo("f") -ge 0) -and ($_.Substring(0, 1).CompareTo("r") -le 0) -and ($IsRunningOnWindows -or -not ($_.Equals("iot"))) }
-    $groups[3] = $allDirs | Where-Object { ($_.Substring(0, 1).CompareTo("s") -ge 0) -and ($_.Substring(0, 1).CompareTo("z") -le 0) }
+    $groups[0] = $testDirs
+    $groups[1] = $testDirs | Where-Object { ($_.Substring(0, 1).CompareTo("a") -ge 0) -and ($_.Substring(0, 1).CompareTo("e") -le 0) }
+    $groups[2] = $testDirs | Where-Object { ($_.Substring(0, 1).CompareTo("f") -ge 0) -and ($_.Substring(0, 1).CompareTo("r") -le 0) -and ($IsRunningOnWindows -or -not ($_.Equals("iot"))) }
+    $groups[3] = $testDirs | Where-Object { ($_.Substring(0, 1).CompareTo("s") -ge 0) -and ($_.Substring(0, 1).CompareTo("z") -le 0) }
     $dirs = $groups[$GroupNumber]
 
-    # Find all the runTest scripts.
-    $scripts = Get-ChildItem -Path $dirs -Filter *runTest*.ps* -Recurse
-    $scripts.VersionInfo.FileName `
-        | Sort-Object -Descending -Property {Get-GitTimeStampForScript $_} `
-        | Run-TestScripts -TimeoutSeconds 600
+    if ($dirs.Count -gt 0)
+    {
+        # Find all the runTest scripts.
+        $scripts = Get-ChildItem -Path $dirs -Filter *runTest*.ps* -Recurse
+        $scripts.VersionInfo.FileName `
+            | Sort-Object -Descending -Property {Get-GitTimeStampForScript $_} `
+            | Run-TestScripts -TimeoutSeconds 600
+    }
 } finally {
     Pop-Location
 }