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.