unittests: ignore nix rpaths

As a necessity nix adds a bunch of rpaths to files, this is unavoidable
do to the way nix package management works. Meson doesn't expect this
however, and fails all rpath tests. To correct this we just ignore any
rpath entries that start with `/nix`.

run_unittests.py

@@ -92,7 +92,7 @@ def chdir(path: str):
         os.chdir(curdir)
 
 
-def get_dynamic_section_entry(fname, entry):
+def get_dynamic_section_entry(fname: str, entry: str) -> T.Optional[str]:
     if is_cygwin() or is_osx():
         raise unittest.SkipTest('Test only applicable to ELF platforms')
 
@@ -106,14 +106,21 @@ def get_dynamic_section_entry(fname, entry):
     for line in raw_out.split('\n'):
         m = pattern.search(line)
         if m is not None:
-            return m.group(1)
+            return str(m.group(1))
     return None # The file did not contain the specified entry.
 
-def get_soname(fname):
+def get_soname(fname: str) -> T.Optional[str]:
     return get_dynamic_section_entry(fname, 'soname')
 
-def get_rpath(fname):
-    return get_dynamic_section_entry(fname, r'(?:rpath|runpath)')
+def get_rpath(fname: str) -> T.Optional[str]:
+    raw = get_dynamic_section_entry(fname, r'(?:rpath|runpath)')
+    # Get both '' and None here
+    if not raw:
+        return None
+    # nix/nixos adds a bunch of stuff to the rpath out of necessity that we
+    # don't check for, so clear those
+    final = ':'.join([e for e in raw.split(':') if not e.startswith('/nix')])
+    return final
 
 def is_tarball():
     if not os.path.isdir('docs'):