[gdb/testsuite] Set language in gdb.ada/minsym.exp

When building gdb using configure flag
--with-separate-debug-dir=/usr/lib/debug, and running test-case
gdb.ada/minsyms.exp, I run into:
...
(gdb) PASS: gdb.ada/minsyms.exp: print some_minsym
print integer(some_minsym)^M
A syntax error in expression, near `some_minsym)'.^M
(gdb) FAIL: gdb.ada/minsyms.exp: print integer(some_minsym)
...
and 2 other FAILs.

This is due to the fact that the language after arriving at the ada main
function turns out to be auto/c.  [ This has been filed as PR25655 -
"Language is not auto/ada in main ada function for exec without debug info". ]

When building gdb without the configure flag mentioned above, we have instead
auto/ada in the ada main function, and all tests pass.

Fix the FAILs by working around PR25655 in the test-case, and manually setting
the language to ada.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/minsyms.exp: Set language to ada.