Rename tests with diagnostics on JVM backend

Remove the "_ir" suffix. It was originally used to distinguish tests on
the JVM IR backend from the old JVM backend ("_old"), but the latter are
removed now.

compiler/testData/diagnostics/testsWithJvmBackend/duplicateJvmSignature/specialNames/defaults.kt

@@ -0,0 +1,8 @@
+// FIR_IDENTICAL
+// TARGET_BACKEND: JVM_IR
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class C {
+    <!CONFLICTING_JVM_DECLARATIONS!>fun `a$default`(c: C, x: Int, m: Int, mh: Any)<!> {}
+    <!CONFLICTING_JVM_DECLARATIONS!>fun a(x: Int = 1)<!> {}
+}