[gdb/testsuite] Fix handling of nr_args < 3 in mi_gdb_test

[gdb/testsuite] Fix handling of nr_args < 3 in mi_gdb_test

The documentation of mi_gdb_test states that the command, pattern and message
arguments are mandatory:
...
 # mi_gdb_test COMMAND PATTERN MESSAGE [IPATTERN] -- send a command to gdb;
 #   test the result.
...

However, this is not checked, and when mi_gdb_test is called with less than 3
arguments, it passes or fails silently.

Fix this by using the following semantics:
- if there are 1 or 2 arguments, use the command as the message.
- if there is 1 argument, use ".*" as the pattern.
- if there are no or too much arguments, error out.

Fix a PATH issue in gdb.mi/mi-logging.exp, introduced by using the command as
message.  Fix a few other trivial-looking FAILs.

There are 11 less trivial-looking FAILs left in gdb.mi in test-cases:
- mi-nsmoribund.exp
- mi-breakpoint-changed.exp
- mi-break.exp.

Tested on x86_64-linux.