Rename/remove some compileKotlinAgainstCustomBinaries tests

"oldJvm*" tests are no longer testing the old JVM backend, so remove
that prefix from them. Also, remove the test "firAgainstOldJvm" because
it is now checking something that cannot NOT work.

compiler/testData/compileKotlinAgainstCustomBinaries/againstUnstable/library/a.kt

@@ -0,0 +1,5 @@
+package lib
+
+class Box(val value: String)
+
+inline fun <T> get(block: () -> T): T = block()

compiler/testData/compileKotlinAgainstCustomBinaries/againstUnstable/output.txt

@@ -0,0 +1,8 @@
+error: classes compiled by an unstable version of the Kotlin compiler were found in dependencies. Remove them from the classpath or use '-Xallow-unstable-dependencies' to suppress errors
+compiler/testData/compileKotlinAgainstCustomBinaries/againstUnstable/source.kt:4:5: error: class 'lib.AKt' is compiled by an unstable version of the Kotlin compiler and cannot be loaded by this compiler
+    get { Box("OK").value }
+    ^
+compiler/testData/compileKotlinAgainstCustomBinaries/againstUnstable/source.kt:4:11: error: class 'lib.Box' is compiled by an unstable version of the Kotlin compiler and cannot be loaded by this compiler
+    get { Box("OK").value }
+          ^
+COMPILATION_ERROR

compiler/testData/compileKotlinAgainstCustomBinaries/againstUnstable/source.kt

@@ -0,0 +1,5 @@
+import lib.*
+
+fun main() {
+    get { Box("OK").value }
+}