Fix testpost.jet: cgi="true" doesn't match __jet.cpp's cgi=="yes" check

$[^session] (a cookie lookup, global.cookies.data[name]) was always
empty because __jet.cpp's cgi keyword check compares against "yes" (a
deliberate choice from an earlier session, not the codebase-wide
true/false convention used by filterblanklines/trimlines/eval etc.) -
testpost.jet had cgi="true", a stale mismatch that predates that
decision, so the entire cgi block (including reading HTTP_COOKIE into
global.cookies) never ran at all. Fixed the test fixture to say
cgi="yes" to match.

Verified live via tests/posttest.sh against the current build:
$[^session] now correctly prints 18fd56a83f (posttest.sh's simulated
HTTP_COOKIE session id), and dump.txt's *** COOKIES *** section shows
session=[18fd56a83f] instead of empty. dump.txt committed with this
fresh, correct output.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/dump.txt

@@ -1,30 +1,8 @@
 *** CGI VARIABLES ***
 *** GLOBAL VARIABLES ***
-=[xxx]
-addition=[8]
-complete=[ABCD;tohex]
-division=[1.666666666667]
-divisor=[8]
-error=[64]
-exprnumbers=[0123456789]
-include=[yes]
-ix=[1]
-lefty=[01234]
-modified1=[ABCD]
-multiplication=[15]
-nested=[64]
-newname=[another container value]
-noeval=[this is the value store in $[%name1].]
-numbers=[0123456789]
-subtraction=[2]
-theexpr=[bcd]
-thename=[this is the value store in .]
-tohex=[tohex]
-varname1=[vardata]
 *** LOCAL VARIABLES ***
-localvar=[This is a container set with '']
-testinclude=[xThis is a container set with ''x]
 *** KEYWORD VALUES ***
-file=[./testinclude.jet]
-name1=[]
+cgi=[yes]
+sessiondir=[.]
 *** COOKIES ***
+session=[18fd56a83f]