test setups: Inherit env when using a test setup

Closes https://github.com/mesonbuild/meson/issues/3525

mesonbuild/build.py

@@ -290,7 +290,7 @@ class EnvironmentVariables:
         return value
 
     def get_env(self, full_env):
-        env = {}
+        env = full_env.copy()
         for method, name, values, kwargs in self.envvars:
             env[name] = method(full_env, name, values, kwargs)
         return env

test cases/unit/2 testsetups/envcheck.py

@@ -0,0 +1,5 @@
+#!/usr/bin/env python3
+
+import os
+
+assert('PATH' in os.environ)

test cases/unit/2 testsetups/meson.build

@@ -14,6 +14,9 @@ add_test_setup('valgrind',
 buggy = executable('buggy', 'buggy.c', 'impl.c')
 test('Test buggy', buggy)
 
+envcheck = find_program('envcheck.py')
+test('test-env', envcheck)
+
 add_test_setup('empty')
 add_test_setup('onlyenv', env : env)
 add_test_setup('onlyenv2', env : 'TEST_ENV=1')