tests: disable incremental cases for now

These were failing on the prior commit. Unfortunately, the fix would
have been relatively complicated, and ties into underlying issues with
the current incremental compilation logic. After discussing this with
Andrew, we agreed that the best course of action is to completely
disable incremental compilation tests for now until it's more mature, at
which point we can re-enable them.

test/src/Cases.zig

@@ -468,6 +468,13 @@ pub fn lowerToBuildSteps(
     incremental_exe: *std.Build.Step.Compile,
 ) void {
     for (self.incremental_cases.items) |incr_case| {
+        if (true) {
+            // TODO: incremental tests are disabled for now, as incremental compilation bugs were
+            // getting in the way of practical improvements to the compiler, and incremental
+            // compilation is not currently used. They should be re-enabled once incremental
+            // compilation is in a happier state.
+            continue;
+        }
         if (opt_test_filter) |test_filter| {
             if (std.mem.indexOf(u8, incr_case.base_path, test_filter) == null) continue;
         }