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 @@ fn line_directive<'line>(
     "only-mips64",
     "only-msp430",
     "only-msvc",
+    "only-musl",
     "only-nightly",
     "only-nvptx64",
     "only-powerpc",