test: Add rundir, rundircmpout and errorcheckdir commands to testlib and run.go
rundir will compile each file in the directory in lexicographic order, link the last file as the main package and run the resulting program. rundircmpout is an related command, that will compare the output of the program to an corresponding .out file
errorcheckdir will compile each file in a directory in lexicographic order, running errorcheck on each file as it compiles. All compilations are assumed to be successful except for the last file. However, If a -0 flag is present on the command, the last compilation will also be assumed successful
This CL also includes a small refactoring of run.go. It was getting unwieldy and the meaning of the run commands was hidden behind argument line formatting.
Fixes #4058.
R=rsc, minux.ma, remyoudompheng, iant
CC=golang-dev
https://golang.org/cl/
6554071