test_*: Disable tests that require setting rlimit on Android

See #7542, it's not totally clear where the problem comes from,
but blanking LD_PRELOAD set by termux seems to fix the problem
(but introduces other issues.

Let's just disable these tests for now.

tests/by-util/test_cat.rs

@@ -98,7 +98,9 @@ fn test_fifo_symlink() {
 }
 
 #[test]
-#[cfg(any(target_os = "linux", target_os = "android"))]
+// TODO(#7542): Re-enable on Android once we figure out why setting limit is broken.
+// #[cfg(any(target_os = "linux", target_os = "android"))]
+#[cfg(target_os = "linux")]
 fn test_closes_file_descriptors() {
     // Each file creates a pipe, which has two file descriptors.
     // If they are not closed then five is certainly too many.

tests/by-util/test_sort.rs

@@ -1090,7 +1090,9 @@ fn test_merge_batch_size() {
 }
 
 #[test]
-#[cfg(any(target_os = "linux", target_os = "android"))]
+// TODO(#7542): Re-enable on Android once we figure out why setting limit is broken.
+// #[cfg(any(target_os = "linux", target_os = "android"))]
+#[cfg(target_os = "linux")]
 fn test_merge_batch_size_with_limit() {
     use rlimit::Resource;
     // Currently need...

tests/by-util/test_split.rs

@@ -1627,7 +1627,9 @@ fn test_round_robin() {
 }
 
 #[test]
-#[cfg(any(target_os = "linux", target_os = "android"))]
+// TODO(#7542): Re-enable on Android once we figure out why rlimit is broken.
+// #[cfg(any(target_os = "linux", target_os = "android"))]
+#[cfg(target_os = "linux")]
 fn test_round_robin_limited_file_descriptors() {
     new_ucmd!()
         .args(&["-n", "r/40", "onehundredlines.txt"])