perf test: Restore sample rate for perf_event_attr

Test "Setup struct perf_event_attr" consists of multiple test cases that
can affect the max sample rate value for perf events. Some test cases
check this value as it should not be lowered under the set minimum for
the given test. Currently, it is possible for the test cases to affect
each other as the previous tests can lower the sample rate, leading to
a possible failure of some of the future test cases as the value is not
restored at any point.

   # 10: Setup struct perf_event_attr:
    --- start ---
    test child forked, pid 104220
    Using CPUID 0x00000000413fd0c1
    running './tests/attr/test-record-C0'
    Current sample rate: 10000
    running './tests/attr/test-record-basic'
    Current sample rate: 900
    running './tests/attr/test-record-branch-any'
    Current sample rate: 600
    running './tests/attr/test-record-dummy-C0'
    Current sample rate: 600
    expected sample_period=4000, got 600
    FAILED './tests/attr/test-record-dummy-C0' - match failure

Restore the max sample rate value for perf events to a reasonable value
before each test case if its value was lowered too much to ensure the
same conditions for each test case.

   # 10: Setup struct perf_event_attr:
    --- start ---
    test child forked, pid 107222
    Using CPUID 0x00000000413fd0c1
    running './tests/attr/test-record-C0'
    Current sample rate: 10000
    running './tests/attr/test-record-basic'
    Current sample rate: 800
    running './tests/attr/test-record-branch-any'
    Current sample rate: 700
    unsupp  './tests/attr/test-record-branch-any'
    running './tests/attr/test-record-branch-filter-any'
    Current sample rate: 10000
    running './tests/attr/test-record-count'
    Current sample rate: 10000
    running './tests/attr/test-record-data'
    Current sample rate: 600
    running './tests/attr/test-record-dummy-C0'
    Current sample rate: 800
    running './tests/attr/test-record-freq'
    Current sample rate: 10000
    ...

Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Radostin Stoyanov <rstoyano@redhat.com>
Signed-off-by: Veronika Molnarova <vmolnaro@redhat.com>
Link: https://lore.kernel.org/r/20241003125136.15918-1-vmolnaro@redhat.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

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