updates on reftests

[rjbou]

• Fix meld reftest: open only with failing ones
• Automatically replace opam bin path, add BASEDIR environment variable
• Add grep -v command, helper instead of just using sed
• On replacement, get the latest group for the end of the match, not the first one (in case groups are defined in regexp) replace all occurrences [hackish]
• Apply grep & seds on file order
• Precise OPAMTMP regexp, hexa instead of 'alphanum to avoid confusion with BASEDIR
• Substitution in regexp pattern (for environment variables)
• Substitution for opam-cat content
• Untie opam root version test from current opam root/version

• some tests update