servlet: Avoid lincheck for normal testing

It is slow, and just tries a bunch of possibilities. Leave it around for
developers to run locally, but it isn't precise enough to run on every
build. This change reduces the build of servlet in isolation by
1 minute. I suspect the gains are larger in a full grpc build because it
is very CPU-intensive; it uses 600% CPU on my 8-thread CPU when running
by itself.

servlet/build.gradle

@@ -8,6 +8,9 @@ description = "gRPC: Servlet"
 def jettyVersion = '10.0.7'
 
 sourceSets {
+    // Testing threading is more of a brute-force test and is very slow. No need to run it as part
+    // of normal suite.
+    threadingTest {}
     // Create a test sourceset for each classpath - could be simplified if we made new test directories
     undertowTest {}
     tomcatTest {}
@@ -39,7 +42,11 @@ dependencies {
     implementation project(':grpc-util'),
             libraries.guava
 
-    testImplementation 'javax.servlet:javax.servlet-api:4.0.1',
+    testImplementation 'javax.servlet:javax.servlet-api:4.0.1'
+
+    threadingTestImplementation project(':grpc-servlet'),
+        libraries.truth,
+	'javax.servlet:javax.servlet-api:4.0.1',
         'org.jetbrains.kotlinx:lincheck:2.14.1'
 
     itImplementation project(':grpc-servlet'),
@@ -74,6 +81,15 @@ tasks.named("test").configure {
     }
 }
 
+tasks.register('threadingTest', Test) {
+    classpath = sourceSets.threadingTest.runtimeClasspath
+    testClassesDirs = sourceSets.threadingTest.output.classesDirs
+}
+
+tasks.named("assemble").configure {
+    dependsOn tasks.named('compileThreadingTestJava')
+}
+
 // Set up individual classpaths for each test, to avoid any mismatch,
 // and ensure they are only used when supported by the current jvm
 def undertowTest = tasks.register('undertowTest', Test) {