test: fix test-module-loading-error for musl

When a compiled library file does not have the proper format,
musl returns the error message ENOEXEC as 'Exec format error' but
glibc returns 'file too short' if the file is under a certain size.

Reference:
http://git.musl-libc.org/cgit/musl/tree/src/errno/__strerror.h#n46

This patch consists of tolerating musl's error.

PR-URL: #3657
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

test/parallel/test-module-loading-error.js

@@ -9,6 +9,7 @@ var error_desc = {
   linux: 'file too short',
   sunos: 'unknown file type'
 };
+var musl_errno_enoexec = 'Exec format error';
 
 var dlerror_msg = error_desc[process.platform];
 
@@ -20,6 +21,10 @@ if (!dlerror_msg) {
 try {
   require('../fixtures/module-loading-error.node');
 } catch (e) {
+  if (process.platform === 'linux' &&
+      e.toString().indexOf(musl_errno_enoexec) !== -1) {
+    dlerror_msg = musl_errno_enoexec;
+  }
   assert.notEqual(e.toString().indexOf(dlerror_msg), -1);
 }