debug/gosym: clean up and modernize pclntab_test

debug/gosym: clean up and modernize pclntab_test

The self tests do not need to build the binary; they won't read it.  The
self tests should work on any ELF system.

Use t.Skip instead of panic.  Use internal/testenv.  Don't worry about a
space in the temporary directory name.

Change-Id: I66ef0af90520d330820afa7b6c6b3a132ab27454
Reviewed-on: https://go-review.googlesource.com/15495
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>