Fix coverage tests on MacOS

Testing Done:
build-support/bin/ci.sh on my mac laptop.

ci green @ https://travis-ci.org/pantsbuild/pants/builds/35870205

Bugs closed: 595

Reviewed at https://rbcommons.com/s/twitter/r/1057/

src/python/pants/backend/python/python_chroot.py

@@ -62,7 +62,8 @@ def __init__(self,
     self._extra_requirements = list(extra_requirements) if extra_requirements else []
     self._platforms = platforms
     self._interpreter = interpreter or PythonInterpreter.get()
-    self._builder = builder or PEXBuilder(tempfile.mkdtemp(), interpreter=self._interpreter)
+    self._builder = builder or PEXBuilder(os.path.realpath(tempfile.mkdtemp()),
+                                          interpreter=self._interpreter)
     self._conn_timeout = conn_timeout
 
     # Note: unrelated to the general pants artifact cache.
@@ -92,7 +93,7 @@ def debug(self, msg, indent=0):
       print('%s%s' % (' ' * indent, msg))
 
   def path(self):
-    return self._builder.path()
+    return os.path.realpath(self._builder.path())
 
   def _dump_library(self, library):
     def copy_to_chroot(base, path, add_function):

src/python/pants/backend/python/test_builder.py

@@ -129,6 +129,12 @@ def _format_string_list(values):
     return '\n\t{values}'.format(values='\n\t'.join(values))
 
   def _generate_coverage_config(self, source_mappings):
+    # For the benefit of macos testing, add the 'real' path the the directory as an equivalent.
+    def add_realpath(path):
+      realpath = os.path.realpath(path)
+      if realpath != canonical and realpath not in alternates:
+        realpaths.add(realpath)
+
     cp = configparser.SafeConfigParser()
     cp.readfp(Compatibility.StringIO(self.DEFAULT_COVERAGE_CONFIG))
 
@@ -138,7 +144,11 @@ def _generate_coverage_config(self, source_mappings):
     cp.add_section('paths')
     for canonical, alternates in source_mappings.items():
       key = canonical.replace(os.sep, '.')
-      cp.set('paths', key, self._format_string_list([canonical] + list(alternates)))
+      realpaths = set()
+      add_realpath(canonical)
+      for path in alternates:
+        add_realpath(path)
+      cp.set('paths', key, self._format_string_list([canonical] + list(alternates) + list(realpaths)))
 
     # See the debug options here: http://nedbatchelder.com/code/coverage/cmd.html#cmd-run-debug
     if self._debug:
@@ -174,6 +184,7 @@ def compute_coverage_modules(target):
     # Take the approach of registering a plugin that replaces the pycov plugin's
     # `pytest_terminal_summary` callback with a noop.
     with temporary_dir() as plugin_root:
+      plugin_root = os.path.realpath(plugin_root)
       with safe_open(os.path.join(plugin_root, 'pants_reporter.py'), 'w') as fp:
         fp.write(dedent('''
           def pytest_configure(__multicall__, config):