Ignore '-C panic=abort' tests for now

We are currently building `libpanic_abort` with the wrong panic
strategy, due to Xargo missing a hack used by `bootstrap`.

tests/compile-fail/panic1.rs

@@ -1,3 +1,4 @@
+// ignore-test: Abort panics are not yet supported
 // error-pattern: the evaluated program panicked
 // compile-flags: -C panic=abort
 fn main() {

tests/compile-fail/panic2.rs

@@ -1,3 +1,4 @@
+// ignore-test: Abort panics are not yet supported
 // error-pattern: the evaluated program panicked
 // compile-flags: -C panic=abort
 

tests/compile-fail/panic3.rs

@@ -1,3 +1,4 @@
+// ignore-test: Abort panics are not yet supported
 //error-pattern: the evaluated program panicked
 // compile-flags: -C panic=abort
 

tests/compile-fail/panic4.rs

@@ -1,3 +1,4 @@
+// ignore-test: Abort panics are not yet supported
 //error-pattern: the evaluated program panicked
 // compile-flags: -C panic=abort