Improve exception message for directories comparison in tests

It was unclear what dirs were compared (e.g when rebuilding and
comparing caches dirs, it's not obvious which dir is "expected" and
which is "actual"). To improve this, compare resulting strings
with the placeholder for a root dir first. If the comparison fails,
then replace the placeholder with an actual directory and call
assertEquals.

build-common/test/org/jetbrains/kotlin/incremental/testingUtils/classFilesComparison.kt

@@ -36,6 +36,7 @@ import org.jetbrains.org.objectweb.asm.ClassReader
 import org.jetbrains.org.objectweb.asm.util.TraceClassVisitor
 import org.junit.Assert
 import org.junit.Assert.assertNotNull
+import org.junit.ComparisonFailure
 import java.io.*
 import java.util.*
 import java.util.zip.CRC32
@@ -71,7 +72,15 @@ fun assertEqualDirectories(expected: File, actual: File, forgiveExtraFiles: Bool
         }
     }
 
-    Assert.assertEquals(expectedString, actualString)
+    if (expectedString != actualString) {
+        val message: String? = null
+        throw ComparisonFailure(
+            message,
+            expectedString.replaceFirst(DIR_ROOT_PLACEHOLDER, expected.canonicalPath),
+            actualString.replaceFirst(DIR_ROOT_PLACEHOLDER, actual.canonicalPath)
+        )
+    }
+
 }
 
 private fun File.checksumString(): String {
@@ -80,6 +89,8 @@ private fun File.checksumString(): String {
     return java.lang.Long.toHexString(crc32.value)
 }
 
+private const val DIR_ROOT_PLACEHOLDER = "<DIR_ROOT_PLACEHOLDER>"
+
 private fun getDirectoryString(dir: File, interestingPaths: List<String>): String {
     val buf = StringBuilder()
     val p = Printer(buf)
@@ -108,7 +119,7 @@ private fun getDirectoryString(dir: File, interestingPaths: List<String>): Strin
     }
 
 
-    p.println(".")
+    p.println(DIR_ROOT_PLACEHOLDER)
     addDirContent(dir)
 
     for (path in interestingPaths) {