test-lib: prevent '--stress-jobs=X' from being ignored

test-lib: prevent '--stress-jobs=X' from being ignored

'./t1234-foo.sh --stress-jobs=X ...' is supposed to run that test
script in X parallel jobs, but the number of jobs specified on the
command line is entirely ignored if other '--stress'-related options
follow.  I.e. both './t1234-foo.sh --stress-jobs=X --stress-limit=Y'
and './t1234-foo.sh --stress-jobs=X --stress' fall back to using twice
the number of CPUs parallel jobs instead.

The former has been broken since commit de69e6f6c9 (tests: let
--stress-limit=<N> imply --stress, 2019-03-03) [1], which started to
unconditionally overwrite the $stress variable holding the specified
number of jobs in its effort to imply '--stress'.  The latter has been
broken since f545737144 (tests: introduce --stress-jobs=<N>,
2019-03-03), because it didn't consider that handling '--stress' will
overwrite that variable as well.

We could fix this by being more careful about (over)writing that
$stress variable and checking first whether it has already been set.
But I think it's cleaner to use a dedicated variable to hold the
number of specified parallel jobs, so let's do that instead.

[1] In de69e6f6c9 there was no '--stress-jobs=X' option yet, the
    number of parallel jobs had to be specified via '--stress=X', so,
    strictly speaking, de69e6f6c9 broke './t1234-foo.sh --stress=X
    --stress-limit=Y'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>