[gdb/testsuite] Accept new complex print style in mixed-lang-stack.exp

[gdb/testsuite] Accept new complex print style in mixed-lang-stack.exp

Since commit 981c08ce72 "Change how complex types are printed in C", we see
these FAILs:
...
FAIL: gdb.fortran/mixed-lang-stack.exp: lang=auto: info args in frame #6
FAIL: gdb.fortran/mixed-lang-stack.exp: lang=c: info args in frame #6
FAIL: gdb.fortran/mixed-lang-stack.exp: lang=c: info args in frame #7
FAIL: gdb.fortran/mixed-lang-stack.exp: lang=c++: info args in frame #6
FAIL: gdb.fortran/mixed-lang-stack.exp: lang=c++: info args in frame #7
...

The problem is that printing of complex types has changed from:
...
d = 4 + 5 * I
...
to:
...
d = 4 + 5i
...
but the test-case still checks for the old printing style.

Fix this by updating the test-case to check for the new style.

gdb/testsuite/ChangeLog:

2020-04-02  Tom de Vries  <tdevries@suse.de>

	* gdb.fortran/mixed-lang-stack.exp: Accept new complex printing style.