runtime(syntax-tests): Support "wait-free" test failure

runtime(syntax-tests): Support "wait-free" test failure

When certain changes guarantee failure for old syntax tests,
opt for faster failure by reducing the number of screendumps
made for each file "page" to be no greater than the assigned
value of a VIM_SYNTAX_TEST_WAIT_TIME environment variable.
(This variable will be ignored and more screendumps may be
made when Make is GNU Make and a parent Makefile is used.)

Barring regressions, and assuming that v9.1.1163~1 succeeds
in providing a correct synchronisation mechanism outside of
"VerifyScreenDump()", and assuming that "readfile()" always
obtains the latest contents written by "term_dumpwrite()" in
"VerifyScreenDump()"; making a single screendump of a file
"page" and following it with a single reading of the written
screendump file should be enough to decide whether to pass
or fail a syntax test.

In addition, re-enable self testing after v9.1.1183~2.

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>