[lit] implement 'env' as shell builtin

[• Tzafrir]

Summary

Add implementation of env command as an internal shell builtin.

[rnk]

This has a few bugs. 'env' should only affect a single command, and it should only affect one command in the pipeline. The env should be reset for the next command in the pipeline. I'm drafting a patch to handle these edge cases, and I'll let you know when it lands.

TestRunner.py

if cmd.commands[0].args[0] == 'cd':
         if len(cmd.commands[0].args) != 2:
             raise ValueError('cd supports only one argument')
         newdir = cmd.commands[0].args[1]
         if os.path.isabs(newdir):
             shenv.cwd = newdir
         else:
             shenv.cwd = os.path.join(shenv.cwd, newdir)
         return 0
+    elif cmd.commands[0].args[0] == 'env':
+        for i in range(len(cmd.commands[0].args)-1):
+            envs=cmd.commands[0].args[i+1].split('=',1)
+            if len(envs)>1:
+                shenv.env[envs[0]]=envs[1]
+            else:
+                #Remove parsed arguments from args list
+                cmd.commands[0].args=cmd.commands[0].args[i+1:]
+                break
 
     procs = []
     input = subprocess.PIPE
     stderrTempFiles = []
     opened_files = []
     named_temp_files = []
     # To avoid deadlock, we use a single stderr stream for piped
     # output. This is null until we have seen some output using