[test][Backtracing] Fix crash message pattern for single threaded mode

With libdispatch executor, `level` function is called on non-main thread
because it does not specify main executor explicitly. Therefore, the
crash message from swift-backtrace says `Thread 1 crashed: ...`.
However, with single threaded executor, all jobs are executed on the
main thread, and main thread always has its name, so the message can be
thread name 'Thread 0 "CrashAsync" crashed:'.

Author: kateinoigakukun

test/Backtracing/CrashAsync.swift

@@ -40,7 +40,7 @@ struct CrashAsync {
 
 // CHECK: *** Program crashed: Bad pointer dereference at 0x{{0+}}4 ***
 
-// CHECK: Thread {{[0-9]+}} crashed:
+// CHECK: Thread {{[0-9]+( ".*")?}} crashed:
 
 // CHECK:  0                  0x{{[0-9a-f]+}} {{_?}}$s10CrashAsync5crashyyF + {{[0-9]+}} in CrashAsync at {{.*}}/CrashAsync.swift:21:15
 // CHECK-NEXT:  1 [ra]             0x{{[0-9a-f]+}} {{_?}}$s10CrashAsync5levelyySiYaFTY0_ + {{[0-9]+}} in CrashAsync at {{.*}}/CrashAsync.swift:29:5
@@ -57,7 +57,7 @@ struct CrashAsync {
 
 // FRIENDLY: *** Program crashed: Bad pointer dereference at 0x{{0+}}4 ***
 
-// FRIENDLY: Thread {{[0-9]+}} crashed:
+// FRIENDLY: Thread {{[0-9]+( ".*")?}} crashed:
 
 // FRIENDLY: 0 {{_?}}$s10CrashAsync5crashyyF + {{[0-9]+}} in CrashAsync at {{.*}}CrashAsync.swift:21:15