Make test harness more portable

[nega0]

This PR supersedes #132 in the interest of a cleaner history.

The intent of this PR is to get the test harness to run on more platforms. It replaces or wraps platform (or age) specific bits with more generic alternatives. The patch is minor, and aside from .gitignore only touches 6 of the test harness files, and the common test Makefile. The tests themselves are all untouched. With these patches, the test harness successfully runs all tests on up-to-date macOS Ventura (and they all pass too).

 .gitignore                        |   53 ++++++++++++++++++++++++++++++++++++++
 tests/bin/test_run                |   11 ++++++-
 tests/bin/test_skip               |    3 +-
 tests/bin/testsuite_exit          |    5 ++-
 tests/bin/testsuite_init          |   31 +++++++++++++++++-----
 tests/common.mak                  |   10 +++----
 tests/gendiffcov/simple/script.sh |    2 -
 tests/lcov/extract/extract.sh     |    2 -
 8 files changed, 99 insertions(+), 18 deletions(-)

• Ignores auto-generated files from the test suite
• Swaps out readlink -e in favor of readlink -f
• Makes call to stat portable via bash's OSTYPE
• Wraps reads from '/proc' with bash's OSTYPE
• Uses 'printf' where we're printing ANSI codes
• Uses the $(realpath ...) builtin
• Changes some shebangs to bash, since we're using bash syntax

[nega0]

Essentially the same as #132 with some minor changes and additions.

[henry2cox]

Excellent - thank you for your contribution. I'm traveling at the moment, but will squash and apply this as soon as I'm back in action. Henry