[backtracing] Mark two tests with REQUIRES (#67074)

The test were enabled even if Backtracing was not enabled in the build
settings. Backtracing is enabled with the build script, but not in the
default CMake configuration.

Author: drodriguez

test/Backtracing/DwarfReader.swift

@@ -4,6 +4,7 @@
 // RUN: %target-run %t/DwarfReader %t/Inlining | %FileCheck %s
 
 // REQUIRES: OS=linux-gnu
+// REQUIRES: backtracing
 
 @_spi(DwarfTest) import _Backtracing
 

test/Backtracing/ElfReader.swift

@@ -13,6 +13,7 @@
 // RUN: %S/Inputs/make-debuglink %t/fib %t/fib-stripped %t/fib.dbg && %target-run %t/ElfReader %t/fib-stripped | %FileCheck %s --check-prefix DBGLINK
 
 // REQUIRES: OS=linux-gnu
+// REQUIRES: backtracing
 
 @_spi(ElfTest) import _Backtracing