To allow testing of fixedbugs/bug345.go in Go,
a new flag -n is introduced. This flag disables setting
of relative path for local imports and imports search path
to current dir, namely -D . -I . are not passed to the compiler.
Error regexps are fixed to allow running the test in temp directory.

This change eliminates the last place where Perl
script "errchk" was used.

Fixes #25586.

Change-Id: If085f466e6955312d77315f96d3ef1cb68495aef
Reviewed-on: https://go-review.googlesource.com/115277
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>