tests: make tests with temporary environment setting portable
With "dash" (as "/bin/sh" on Debian GNU/Linux), command execution in
"ENV=val foo bar" style doesn't work as expect in test script files,
if "foo" is user-defined function: it works fine, if "foo" is existing
commands like "hg".
09049042ab99 introduced tests for HGPLAIN and HGPLAINEXCEPT into
test-revset.t, and all of them are in such style.
This patch doesn't:
- add explicit unsetting for HGPLAIN and HGPLAINEXCEPT
they are already introduced by 09049042ab99
- write assignment and exporting in one line
"ENV=val; export ENV" for two or more environment variables in one
line causes failure of test-check-code-hg.t: it is recognized as
"don't export and assign at once" unfortunately.