tests: monitor: Fix for out-of-path call

tests: monitor: Fix for out-of-path call

When called from another directory without specifying test cases, an
incorrect regexp was used to glob all tests and no test was run at all:

| # ./tests/monitor/run-tests.sh
| echo: running tests from file *.t
| ./tests/monitor/run-tests.sh: line 201: testcases/*.t: No such file or directory
| monitor: running tests from file *.t
| ./tests/monitor/run-tests.sh: line 201: testcases/*.t: No such file or directory
| json-echo: running tests from file *.t
| ./tests/monitor/run-tests.sh: line 201: testcases/*.t: No such file or directory
| json-monitor: running tests from file *.t
| ./tests/monitor/run-tests.sh: line 201: testcases/*.t: No such file or directory

Fixes: 83eaf50c36fe8 ("tests: monitor: Become $PWD agnostic")
Signed-off-by: Phil Sutter <phil@nwl.cc>