test: functional: drop unused --keepcache argument

At the time this was added in  #10197, building the test cache took 21
seconds, as described in that PR, but this is no longer true, as
demonstrated by running the functional test framework with and without
the --keepcache arguments on master prior to this commit:

```
hyperfine   --warmup 1  --export-markdown results.md --runs 3 \
    -n 'without --keepcache'    './build/test/functional/test_runner.py -j $(nproc)' \
    -n 'with --keepcache'       './build/test/functional/test_runner.py -j $(nproc) --keepcache'
```

| Command               | Mean [s]       | Min [s] | Max [s] | Relative |
|:----------------------|---------------:|--------:|---:|---:|
| `without --keepcache` | 76.373 ± 3.058 | 74.083  | 79.846  | 1.00 |
| `with --keepcache`    | 77.384 ± 1.836 | 75.952  | 79.454  | 1.01 ± 0.05 |

As a consequence, this argument can be removed from the test runner and
this also has the benefit of being able to use an RAII-like
`tempfile.TemporaryDirectory` instead of having to clean up the cache
manually at the end of test runs.

bitcoin/bitcoin#10197: https://github.com/bitcoin/bitcoin/pull/10197

test/functional/test_runner.py

@@ -415,7 +415,6 @@ def main():
     parser.add_argument('--extended', action='store_true', help='run the extended test suite in addition to the basic tests')
     parser.add_argument('--help', '-h', '-?', action='store_true', help='print help text and exit')
     parser.add_argument('--jobs', '-j', type=int, default=4, help='how many test scripts to run in parallel. Default=4.')
-    parser.add_argument('--keepcache', '-k', action='store_true', help='the default behavior is to flush the cache directory on startup. --keepcache retains the cache from the previous testrun.')
     parser.add_argument('--quiet', '-q', action='store_true', help='only print dots, results summary and failure logs')
     parser.add_argument('--tmpdirprefix', '-t', default=tempfile.gettempdir(), help="Root directory for datadirs")
     parser.add_argument('--failfast', '-F', action='store_true', help='stop execution after the first test failure')
@@ -565,9 +564,6 @@ def main():
     check_script_list(src_dir=config["environment"]["SRCDIR"], fail_on_warn=fail_on_warn)
     check_script_prefixes()
 
-    if not args.keepcache:
-        shutil.rmtree("%s/test/cache" % config["environment"]["BUILDDIR"], ignore_errors=True)
-
     run_tests(
         test_list=test_list,
         build_dir=config["environment"]["BUILDDIR"],
@@ -598,11 +594,6 @@ def run_tests(*, test_list, build_dir, tmpdir, jobs=1, enable_coverage=False, ar
         # pgrep not supported
         pass
 
-    # Warn if there is a cache directory
-    cache_dir = "%s/test/cache" % build_dir
-    if os.path.isdir(cache_dir):
-        print("%sWARNING!%s There is a cache directory here: %s. If tests fail unexpectedly, try deleting the cache directory." % (BOLD[1], BOLD[0], cache_dir))
-
     # Warn if there is not enough space on the testing dir
     min_space = MIN_FREE_SPACE + (jobs - 1) * ADDITIONAL_SPACE_PER_JOB
     if shutil.disk_usage(tmpdir).free < min_space:
@@ -614,7 +605,8 @@ def run_tests(*, test_list, build_dir, tmpdir, jobs=1, enable_coverage=False, ar
     # a hard link or a copy on any platform. See https://github.com/bitcoin/bitcoin/pull/27561.
     sys.path.append(tests_dir)
 
-    flags = ['--cachedir={}'.format(cache_dir)] + args
+    cache_tmp_dir = tempfile.TemporaryDirectory(prefix="functional_test_cache")
+    flags = [f"--cachedir={cache_tmp_dir.name}"] + args
 
     if enable_coverage:
         coverage = RPCCoverage()
@@ -631,7 +623,7 @@ def run_tests(*, test_list, build_dir, tmpdir, jobs=1, enable_coverage=False, ar
             sys.stdout.buffer.write(e.output)
             raise
 
-    #Run Tests
+    # Run Tests
     job_queue = TestHandler(
         num_tests_parallel=jobs,
         tests_dir=tests_dir,