tests: Fix duplicated-path-in-error fail with musl

musl's dlopen returns a different error than glibc, which contains the
name of the file. This would cause the test to fail, since the filename
would appear twice in the output (once in the error from rustc, once in
the error message from musl). Split the expected test outputs for the
different libc implementations.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

src/tools/compiletest/src/directives.rs

@@ -973,6 +973,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "only-mips64",
     "only-msp430",
     "only-msvc",
+    "only-musl",
     "only-nightly",
     "only-nvptx64",
     "only-powerpc",

tests/ui/codegen/duplicated-path-in-error.musl.stderr

@@ -0,0 +1,2 @@
+error: couldn't load codegen backend /non-existing-one.so: Error loading shared library /non-existing-one.so: No such file or directory
+

tests/ui/codegen/duplicated-path-in-error.rs

@@ -1,8 +1,15 @@
+//@ revisions: musl gnu
 //@ only-linux
+//@ ignore-cross-compile because this relies on host libc behaviour
 //@ compile-flags: -Zcodegen-backend=/non-existing-one.so
+//@[gnu] only-gnu
+//@[musl] only-musl
 
 // This test ensures that the error of the "not found dylib" doesn't duplicate
 // the path of the dylib.
+//
+// glibc and musl have different dlopen error messages, so the expected error
+// message differs between the two.
 
 fn main() {}