[Gradle, Wasm] Fix proxy for karma wasm tests on windows

^KT-62128 fixed

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/js/testing/karma/KotlinKarma.kt

@@ -354,7 +354,7 @@ class KotlinKarma(
                     createLoadWasm(npmProject.dir, file).normalize().absolutePath
                 )
 
-                config.proxies["/${wasmFile.name}"] = wasmFileString
+                config.proxies["/${wasmFile.name}"] = basify(npmProjectDir, wasmFile)
 
                 config.customContextFile = npmProject.require("kotlin-test-js-runner/static/context.html")
                 config.customDebugFile = npmProject.require("kotlin-test-js-runner/static/debug.html")
@@ -396,7 +396,7 @@ class KotlinKarma(
             escapeTCMessagesInLog = isTeamCity.isPresent
         )
 
-        config.basePath = npmProject.nodeModulesDir.absolutePath
+        config.basePath = npmProjectDir.absolutePath
 
         configurators.forEach {
             it(task)
@@ -606,6 +606,15 @@ class KotlinKarma(
     }
 }
 
+// In Karma config it means relative path based on basePath which is configured inside Karma config
+// It helps to get rid of windows backslashes in proxies config
+// `base` is using because of how Karma works with proxies
+// http://karma-runner.github.io/6.4/config/configuration-file.html#proxies
+// http://karma-runner.github.io/6.4/config/files.html#loading-assets
+internal fun basify(npmProjectDir: File, file: File): String {
+    return "/base/${file.relativeTo(npmProjectDir).invariantSeparatorsPath}"
+}
+
 internal fun createLoadWasm(npmProjectDir: File, file: File): File {
     val static = npmProjectDir.resolve("static").also {
         it.mkdirs()

libraries/tools/kotlin-gradle-plugin/src/test/kotlin/org/jetbrains/kotlin/gradle/targets/js/testing/karma/KotlinKarmaTest.kt

@@ -35,4 +35,17 @@ class KotlinKarmaTest {
             loadWasm.readText().trimIndent()
         )
     }
+
+    @Test
+    fun checkBasify() {
+        val npmProjectDir = createTempDirectory("tmp")
+        val executableFile = npmProjectDir.resolve("kotlin/main.wasm")
+
+        val based = basify(npmProjectDir.toFile(), executableFile.toFile())
+
+        assertEquals(
+            "/base/kotlin/main.wasm",
+            based
+        )
+    }
 }