Tests: Disable test

Test `roundTrippingUInt128` appears to be causing a stack dump in an OSS
build on linux.  Disable this test for now until we can properly get
this resolved.

rdar://154155359

Author: bkhouri

Tests/FoundationEssentialsTests/JSONEncoderTests.swift

@@ -1433,18 +1433,29 @@ private struct JSONEncoderTests {
             }
         }
     }
-    
-    @Test(arguments: [
-        UInt128.zero,
-        1,
-        0x0000_0000_0000_0000_7fff_ffff_ffff_ffff,
-        0x0000_0000_0000_0000_8000_0000_0000_0000,
-        0x0000_0000_0000_0000_ffff_ffff_ffff_ffff,
-        0x0000_0000_0000_0001_0000_0000_0000_0000,
-        0x7fff_ffff_ffff_ffff_ffff_ffff_ffff_ffff,
-        0x8000_0000_0000_0000_0000_0000_0000_0000,
-        .max
-    ])
+
+    private var isLinux: Bool {
+        #if os(Linux)
+        true
+        #else
+        false
+        #endif
+    }
+
+    @Test(
+        .disabled(if: isLinux, "This test is not applicable on this platform"),
+        arguments: [
+            UInt128.zero,
+            1,
+            0x0000_0000_0000_0000_7fff_ffff_ffff_ffff,
+            0x0000_0000_0000_0000_8000_0000_0000_0000,
+            0x0000_0000_0000_0000_ffff_ffff_ffff_ffff,
+            0x0000_0000_0000_0001_0000_0000_0000_0000,
+            0x7fff_ffff_ffff_ffff_ffff_ffff_ffff_ffff,
+            0x8000_0000_0000_0000_0000_0000_0000_0000,
+            .max
+        ]
+    )
     func roundTrippingUInt128(u128: UInt128) {
         _testRoundTrip(of: u128)
     }